got an extra engine off my last buy but no idea if they're any good to use

 

Housings, you just want to check if there are any chips on the edges, or chrome flaking. You also want to see if there is a groove made by the 3rd part of the OEM apex seal (not supposed to be there, housing surface is supposed to be smooth).

For the rotors, check if there is any copper showing through on the surface of the bearing. If the babbit coating is less than 80% on the bearing (20% or more copper), it is recommended that the bearing be replaced in that rotor. This is because that shows the eccentric shaft was in contact with the bearing while the engine was in use. Usually there is a thin film of oil between the e-shaft and the bearing that stops this from happening.

Also, check if the apex seal grooves are worn out or chipped. Say if you are using a 2MM apex seal, and the groove was originally manufactured for it, you should check if the groove has bead out. If there is too much of a gap between the grooves and the apex seal you should not use it.

Check for carbon buildup and the overall cleanliness of the engine. That's not a problem though as it can be cleaned/scrapped off, it just takes some elbow work and a parts cleaner.