Yeh, Classic BattleTech rocks on toast. But it'd probably take more than a half hour or so to play; still, it rocks.
I haven't tried the Mecha d20 SRD yet, I've only read a bit of it before, but it's still available for download from the Guardians of Order website I think. Though the company is defunct now, last I heard the website was still online. To make full use of the Mecha d20 SRD, however, you might also need to download the Anime d20 SRD from that same website. The latter is the Open Game Content name for the rules from Big Eyes, Small Mouth d20 (GoO's conversion of the original BESM rules to d20), and I don't recall what fellow EN Worlders generally thought of them (though I do recall many liking the Mecha d20 SRD).
I forget what published book the Mecha d20 SRD is drawn from, but it's a related book published by Guardians of Order. Be warned though, it's a fairly comprehensive system and rather different from D20 Modern, I think, from what I read of it before deciding I wanted to avoid that degree of granularity/complexity. The Mecha d20 SRD's rules are designed for use in making just about any kind of vehicular device; be it a walking mecha, a high-tech motorcycle, a fighter jet, a hovertank, or whatever. I don't recall if it's got any rules for making robots, but you can check for yourself if you download it.
You may or may not like to use the Mechamorphosis book, from Fantasy Flight Games, published in their Horizons line of one-shot books. But that's more for playing Transformers-style robots and duking it out Autobot-versus-Decepticon (Exile vs. Tyrant in Mechamorphosis description) style. I really need to pick this up soon, but I kept forgetting earlier at the only times I had the cash to spare for an RPG book.
You could instead try D20 Modern + D20 Future + D20 Future Tech. These are from Wizards of the Coast. D20 Future has a Mecha chapter near the back, between the Starships chapter and the Robots chapter, with rules for building and running mecha. They operate generally more like powered armor in D20 Future, for reference, rather than the walking tanks of BattleTech. The mecha in D20 Future provide their pilot (and any copilots, if you bother to waste space on copilot cockpits, though a copilot can sometimes help) with 100 or more extra HP (depending on size; Gargantuan or Colossal mecha give several hundred HP) as well as the mecha's own hardness, equipment bonus to Defense (replacing any normal armor bonus to Defense, generally; Defense is D20 Modern's term for Armor Class).
D20 Future Tech has a mecha section near the back too (it's a much thinner, softcover book, whereas D20F and D20M are thick hardcovers). D20FT also has a brief chapter on mixing character-scale combat with mecha/vehicle/starship combat scales, and other mixing of those types (like mecha as the focus, or starships as the focus, with EV suits and forced boarding, and such). Basically, D20 Future Tech has some extra Mecha weapons, armors, sensors, leg-replacement options, other propulsion units, and other stuff (like strength enhancement, speed boosters, camouflage systems, chaff and flares, and metabot links).
The metabot link is something you can put into one of a mecha's equipment slots when designing it, to allow it to link up with other mecha possessing the metabot link, and form a bigger super-mecha. Basically like in the Transformers cartoons/anime, or the Delta Veritech I think in Robotech (I think it was the Delta version that separated into something like three separate mini-vehicles).
D20 Future Tech also has some alternate mecha frames, like the scout walker frame, the sentinel frame, and a few others, which grant some slightly different basic stats for the mecha and some different slots (for instance, the scout walker frame removes the mecha's arms but gives it some extra leg equipment slots, if I recall correctly). Similarly, while D20 Future has something like 4 sample mecha statted up, D20 Future Tech has an additional 5 or 6 sample mecha (both have mecha of varying sizes; D20FT has different ones that use the alternate frames, along with illustrations of each, whereas D20F only has an illustration for the Myrmidon mecha).
However, that said, you should probably look around for Rodney Thompson's (Moridin on the EN World forums, IIRC; or maybe that was JD Wiker's username around here? Nah, I think it is Rodney) table of D20 Future Tech weapon stats. See, the table that was supposed to go in the D20 FT chapter on mecha was incidentally left out when it went to print, so there aren't complete stats for the mecha weapons in D20 FT (most have nearly-complete stats in their description, but a few details are left out; in one or two weapons' cases, their actual damage and such is left out, unfortunately).
I keep forgetting to ask around here for Moridin to re-post (or re-direct us to) the table with those missing stats. And to ask again how Psi Tech is supposed to work (D20 FT included several PsiTech gadgets and devices in its first chapter, but never explained how they're supposed to be used exactly). I never really got an answer on that second question last time we EN Worlders asked around; I do know that the former question (mecha weapons table) did receive an answer around here, but I just don't recall where or how to find the missing table.
Maybe it's because we tended to start on the far, opposite ends of the battlefield, and used maps with lots of different terrain obstacles. 
