bulletmeat
Adventurer
I'm sure I will get thrown under the bus for this but you could look at Castles & Crusades. PHB is a free PDF, rules are pretty straight forward (B/X style attributes, class level + mod for class related proficiencies, EASILY modded w/Basic to 2e adventures). Rule have been primarily the same since 2004. I've used the C&C PHB, 1e DMG, & 2e monster manual w/practically no work converting.
And while some don't like the siege engine it's based on, that engine is very easily modded.
And while some don't like the siege engine it's based on, that engine is very easily modded.