Engine differences

Is there any differences in the early model trx300 fourtrax engines compared to a later model trx300's? I have a friend needing an engine for his '95 trx300 4x4 and he has found a parts bike that looks like an early model, '88 to maybe a '91. looking at the pics of the parts bike, I can tell right off hand the exhaust is different.
Will the early engine bolt up to his later model frame?