Building a pair of Deutschlands

I'm typically an advocate of sticking with brushed for most applications. I value reliability and performance over innovation. The drive/pump setup you have looks fine. There are advantages to having some excess room on the throttle adjustment (ability to get reverse speed very close to forward speed). I think the old motors will give you better performance over the lower RPM motors, the boat will start feeling like the 'little engine that could' while reversing direction with lower RPM brushed motors (at 6ish V specifically). At least get the boat to a point where everything works in almost all battles, it can stay on the surface, and hopefully give more damage than it takes, then if you want to change (and reduce reliability) look into it at that point. Just my general suggestions for maximizing satisfaction with boat performance

If you want to go brushless and use the existing gearbox, the NTM series of motors on hobbyking are pretty common. A 35mm can will give you way more power than you need, but is a drop in for the 550 mounting. A 28mm can would be more appropriate, but has different mounting hole spacing (19mm). Getting to direct drive would be better, but more work. The northeast folks use the hobbyking car ESCs available in a range of current ratings. Its reliability is good for brushless, but failure rates will be higher than the brushed hobbywing Esc you currently have https://hobbyking.com/en_us/hobbykingr-tm-brushless-car-esc-45a-w-reverse.html

For the pump, a 28mm motor in the 2000 - 2500kV range would work. You can get the 19mm motor plate from BC for an easy change, just remember your pump laws where considering cost/benefit and discharge rate/Ah limitations of batteries. Getting above the brushed pump flow rate will certainly cost you in amperage, and really shouldn't be needed on a pre-dread. It will need a separate ESC as well. The lower bearing will need TLC. I'd say geometry/weight is the only benefit there, with higher cost, more maintenance, less reliability

 

Bearings are cheaper than a whole new motor. 10 replacement bearings from China cost $20 including shipping. Brass rod to tap the blown bearing out and your new again.
Plus brushless will run without balls in the race (noisy) , insert tasteless joke here.

 

I like the sk3 HK outrunners. I think they are an excellent cross section of quality and price.