ernest2,
As Tachikaze suggested, a geared motor(s) may not be necessary and a direct drive system may work just fine. A 'Grand Banks' isn't a 'speed' boat although they are fairly fast in the 'full sized' versions. Scale speeds are really much slower than what you'd think (or want), don't worry too much about finding a large motor(s). It's 'better' to have a motor that's too large than too small though, then just be 'easy' on the throttle. The 'extra' speed/power can get you out of some situations (and if you can keep off the throttle you've got more will power than I do LOL!), and it can also get you INTO those situations if you're not careful! Two '540's should be more than enough for your 'Grand Banks'... I'd think.

Making your own drive shafts and stuffing boxes is certainly an option, lots of people do it. You might have to order the bearings (no idea what you may find available there), but that shouldn't be much of a problem. Making allowances for greasing/oiling the drive shaft is something you might think about. Having to disassemble half the boat to get to the stuffing box is a real P.I.T.A. After the first time, you'll know exactly what I mean!

"Scrounging", or 'dumpster diving' can be a very nice source of various parts/material. You'd be amazed at what people throw away. Small appliance service centers, RadioShack, Sears, whatever, are gold mines for small motors, brass, plastic sheets, gears, and 'stuff' you didn't even know you needed! If you have children/grandchildren/neighbors with kids, raid their toy boxes. You wanna be careful, kids are vicious when you mess with their toys!
- 'Doc