Well, the 3974 could fit, as long as the screw-holes are 25mm apart. The 2050 is a good choice too imho if You plan on using 3s.
I mentioned the 36mm because people tend to mix-up water-cooled 28xx en 36xx motors, the can
including the cooling mantle of a 28xx motor often looks like a 36xx. 28xx motors are just too feeble to power a 28mm jet. They'll burn themselves out in no-time short.
If the holes are apart more than 25mm, you probably have to purchase a separate adapter for 800-type motors from Kehrer Modellbau, and with postage to Oz, that could run as expensive as buying a new motor. Unless the holes in a 39xx are the same as in a brushed 700, which I do not know. Haven't got a 39xx around, I can't check. And I haven't used 700's in, oh, 15 years, and donated the last one to a friend, who promptly cooked it...
(the last boat I used a turbo 700 in, was a Graupner Key-Biscayne on 14 cells with a 50mm carbon hydro-prop. For a time the fastest boat in the vicinity. Sold it, and the kid who bought it, planted it in a rather solid harbour wall 
Yup, I warned him that 46km/h on water was pretty fast, and that braking, even with reverse, takes some time..... I believe I still got a vhs (oh, yeah, baby...) with the "Sea-Hulk". A video that starts with a nervous, but fast Graupner Cobra, then a sonorous low buzz can be heard, and the Key-Biscayne majestically overtakes the Cobra like it's standing still, Leaving it solidly in it's wake..)
The 2050, with it's 4-poles and 1800 watts is gonna be pretty darned quick on 3s, much faster than mine on 2s, and FLY on 4s. It should run pretty long on the 5000ah pack too. Just do. NOT. fully rev the boat out of the water when trying 4s. It might sound cool, like an angry F1 car, but it WILL wreck the prop and chamber of the jet. (Don't ask
)
and if the
2200kv 3974 fits? (Which I think it will) Well.. Hoo boy, hold-on to Your b*tt.. :P that's basically a small-block 1/8 buggy motor. Torque-a-plenty.