Be weary....
No....I'd be weary because he's already told you that it's got a more powerful engine in it than it came with stock. The 1997 GTI came stock with the 717cc (85 hp) engine....And one with 40 hours on it?....that's all but impossible.
If it's a 97, tell him your interested but because the price is a bit high, you want to take it to your local dealer where they can put it on an MPEM programmer and read the actual hours the ski has on it. If he says no...then he's hiding something.
I also recommend that someone always do a compression test on any engine powered craft. Especially one that's over 10 years old. You should see pressure over 100 psi and it should have no more than 8 PSI between the two pistons....
Last, if you live in Maine and it was not winterized, there's a good possibility that it's got a cracked head or other related damage from freezing.
If your not very mechanical inclined, I"d probably stay away from it.