generally .12-.18 motors are small blocks and .21-.32+ are big blocks. most small blocks will be compatable with each other with minor modifications. and the same would apply for big blocks. but... some engines have side or rear exhaust which will determine which exhaust manifold you will need to buy. also some carbs as was mentioned earlier are different. some are rotory and some are slide. this will affect which linkage you will need to make every thing work. those are 2 big thing you will need to watch for when getting a new motor. the easiest swap will be if you have a small block then replace it with a small block and the same with big blocks. if you have a slide carb get one with a slide carb, and if you have a rear exhaust get one with a rear exhaust. if you start changing these things around then you will have more trouble getting it to fit properly. ofcourse there are exceptions to every rule and a dremel is a very handy tool!lol!