After working in a small engine shop, and having to fix people severely neglected, destroyed pieces of trash. I prefer to not even waste my time opening up somebody's random junk engine. (unless of course its rare, or i have a special vested interest). If your gonna do it right, your gonna have to replace most parts to keep from constantly breaking down and having to tear it apart again. I'll gladly pay the 71 dollars again (90 with 2 year warrenty) for piece of mind in knowing that i took good care of it from day one. And if it breaks, do NOT walk into the shop, do NOT pick up a wrench, go to HF set it on the counter and DO collect your brand new engine.
I've got 3 briggs and and 2 lawn boys that i got for free and cleaned the carb and got running. But i don't feel it even worth the time to build something, because if i do it cheap i'll constantly have to fix it, and if i do it right it'll cost money, and be worth putting a nice engine on it (or if i run across a unique engine, spend the money to fully rebuilt it)
Plus i consider my time valuable, and having to deal with greasy, rusted engines with stale gas and stripped bolts, is not my idea of a good time.