Eh, I'd make it a simpler exercise. Target the whole thing to 18 levels of play, not 30 (yes, I know BECMI theoretically had no cap, etc, but realistically power progression past level 18 is minimal and even if you tacked on an 'immortal' rules beyond 18th it is very different and needn't rely on the lower level power curve).
Now, you can jettison anything that conceptually wasn't relevant to BECMI. So you don't need ability boosts. You get rid of half level, and really you're just left with magic items and that's about it. Arguably you could convert those to a 1/3 level to-hit bonus, and the same for defenses and ditch enhancement. In that case your system works reasonably well, if you plug in the right numbers for hit point and damage progression.
Of course now you want to decide what level range a given monster has SOME relevance over. Well, it is going to be pretty good! Your level 1 orc that say hits AC 15 on a 10 is going to hit AC 21 on a 15, and that's around what your level 18 PC will have. You may even need to trim that back a bit, lets say you want said orc to be BARELY relevant at level 18, so say he hits AC 21 on a 19, then back calculating he hits AC 15 on a 13, and that can be your basic level 1 starting out with limited gear "I'm a fighter" AC (lets say that's Chain and Shield, So Chain is 14, leather and shield is 13, leather is 12, shield is 11, bare assed nekked is a 10, just like in 4e! Going the other way 'Splint Mail' is 16, with a shield 17, plate is 18, plate +shield is 19. So, we have a little bit of a gap there at the top where presumably our top AC is going to hit 25 eventually and price our level 1 orc out of the game, but he can hit even the sturdiest fighter with a 20 at least up to level 9 and presumably he can hit 'squishies' far past that, so he's not totally irrelevant. We could also get rid of 'Splint' armor, which IIRC didn't even exist in Basic anyway, and now all of a sudden our top AC is 23 and said orc can hit up to level 15, which is not far off from our goal.
Now, as far as damage and hit point progression goes... The high level guys are clearly hard for these level 1 monsters to even hit, so how many hit points do we really need? Lets see... If we give our fighter CON + 5 hp/lvl lets just say at 15th level he'll have around 85 hp. If an orc does avg 5 damage on a hit, that means 3 hits to kill a level 1 fighter, which is not far off from Basic (it was probably more like around 2). Hitting on a 20 means to do 85 damage would require 18 hits, or 360 rounds of attacks. Our orc will truly be pretty trivial, probably about as trivial as in Basic at the same point in progression. A huge MASS of orcs, say 200 of them, with bows, could still be quite threatening if they all shot at you of course.
More realistically lets say said orc takes on a level 6 fighter, and say by then he's equipped to have an AC of 18 (1 for level and wearing plate and shield). Now said orc can hit on a 16 and the fighter has around 40 hit points. 5 orcs will hit once a round for 5 damage, so he'll last 8 rounds against them. Of course he'll kill them before they do more than about 20 damage to him, but that still makes them usable mooks and not to be laughed at.
Of course your combat system can be cribbed to any degree you want from Basic. It could be whole-cloth with suitable updates for the 4e-style number engine. Nothing really remarkable there, you can make a usable, if not brilliant, combat system that plays out pretty close to exactly like the Basic one. You could do the same thing for magic of course, nothing remarkable there. In fact there's really not a whole lot more to be said. Your monsters will need stats since pretty much anything except combat is all stat-driven, but you can still boil them down to a line of text per monster in your notes and crack the book open if someone decides to arm-wrestle an orc.
Skills you'll be ignoring, presumably. Now you could of course go back to the barbaric "some random different kind of die for everything" of Basic for stuff like thieves but I see no need. Just give them d20 with a DEX mod based thief abilities. You can always tack on any other little bonus for other PCs to do specific things that you want.
