Buying A Used Engine

what should I look for in an engine when I buy it, all i know is check for play in the crank, wondering what other signs I would look for.

Also if going from a typhoon non rolling nut to a rolling nut (the engine i want to buy) would i need different loom or will the same loom work

 

look up the zorst port for signs of blow by on the piston, and build up of carbon ontop of it

 

i dont really need the piston or the cylinder head as im going to fit my 172 cylinder and piston onto it, so does that mean nothing else would have to be looked out for?

well i know to look for cracks on the case and that but not much else