Brewing the Game Fantastic...

My favorite feature of gaming in the late 1970s (aside from the manual type and amateur aesthetic) was the rules-are-just-a guide mentality. We were playing D&D, a common tongue known by all, with each campaign a quaint regional dialect. From a thick Boston accent to lazy southern drawl, we spoke the same language, but it was different. The referee could mix and match what they wanted, even taking from other games. Decades later, our so-called classic catalog tries to do the same. You can eat 'em RAW or get creative...

RAW (rules-as-written) is fine; but you can add or change anything to suit your style, and guess what? Our stuff was deliberately made for this. Try these: 

Barons of Braunstein with Pits & Perils' armor system (and for added detail, assign bonus hits per Gaming Primitive). Oh, and maybe its magic items as rare artifacts...

Blood of Pangea using Barons of Braunstein's literacy system and/or Gaming Primitive's dual hit point/skill point approach for versatile heroes and gameplay...  

Diceless Dungeons, but with Barons of Braunstein's dicey combat and modifiers.

Pits & Perils, with Barons of Braunstein's mass combat and siege rules with outcomes tied to specific actions undertaken by the characters on some secret mission...

Each of these has its own feel; which is to say, Barons of Braunstein is experienced quite differently, thematically and mechanically, than say, Pits & Perils. But they snap together like Lego blocks with minimal effort. They have a shared core. It isn't quite an engine (I reject mechanical builds and video-game comparisons); but make your system simple enough and focus on content (monsters and the like), and you get something very much like an engine, although a common tongue with many regional dialects is my preferred comparison.