I think for $300 if you can get the motor running, you'll probably at least come close to covering the cost if the boat itself isn't salvageable. Unfortunately I'm not familiar with the critchfeild montego 17. I just bought an old beater fixer upper trimaran too and there are lots of things to consider when buying a "project boat". Depending on how much work it needs you have to understand that it may end up costing more than the boat may be worth when you get it fixed up. That said, it can be very satisfying bringing an old boat back to life, particularly as a father and son project. And when you finish, you will certainly know your boat intimately. The journey can be just as satisfying as the destination.
Are the hulls wood? Is there any rot in the hulls or structure? If so, it might be more of a rebuild from scratch than just a fixer upper. Did it come with good sails and rigging? If you can post pictures and more information on this site, I’m sure you’ll get more detailed information from others who are more experienced than me.
I just bought a fixer upper tri to (for less than a hundred bucks). I figure I’ll spend many many times that and countless hours to get it back into any kind of serviceable condition. There is something very gratifying about bringing an old boat back to life even if it may not be the most practical way to go. Good luck!