I can't answer the question of who has the best engine kit, but I can tell you who to get some qaulity parts. Take a look at the manic mechanic parts.htm, he's got some cool stuff, I bought a clutch actuator from him and was blown away by it, his prices are reasinable too.
ON another note, I purchased a head stud kit from sick bike parts, they sent me 4 cut lengths of allthread, If I wanted allthread I would have gone to home depot, they didn't even cut the lenghts right!