If it's a new engine find out who put it in. Hopefully it was a shop, if so go talk to them. See why the engine was replaced and get a feel for if they are a shop or dealership that would do a good job.
Also, be real thorough about checking the frame and such. 8 year old sled with a new engine probably means he blew the first one up, or put a shit ton of miles on it, which means there might be issues with others things besides the engine. See if you can get the cylinders checked, look at the carbides etc.