I don't know for certain, but I'm pretty sure it's the same as the Buick/Olds engine. 215 horsepower should do pretty well for an 17.5 foot boat, even with a jet drive I would think.
I do know the original manufacturer actually had started with smaller engines in these boats and went to larger ones as they sold them to get better performance. I'm sure the jet could handle it.
If you look at the videos I posted earlier in the thread showing the boat in Kelowna, it seems like it does ok for speed. I'd expect maybe 35-40 mph, and I've read that other owners have used them to pull up 2-3 skiers at a time since you can rev the engine full before engaging the jet.
I've seen plenty of the newer jet boats with the Olds 455 engines and jacuzzi drives, and frankly I don't like anything about 'em. Most of them are impractical go-fast types with metal flake paint and engines that look like they're big enough to plane on their own
I'd rather have something like this... vintage, fun, sounding cool.
Erik
Edit: Thinking about it, if I did replace the engine, I'd probably put in a regular 5.7 vortec chevy small block. Looking at the drive train I'm certain I could adapt almost any engine to it, it's really simple.