The prop fitted to the model when I bought it is a 50x two bladed, there is sufficient clearance to go to 55mm, but that's the lot. I am not sure what voltage the builder ran the MFA 850 on, I have yet to untangle the battery to ESC cables, but it seems that he used 4 packs. The MFA 850 is a 12 volt motor according to orders still advertising it, and can be run at up to 11 amps.
If I use a 500 kv motor and 6s LiPo pack, the theoretical RPM would be approx 11,000. I suspect in the real world it would be between 9 & 10k on the water on the rare occasions I used full throttle.
Looking at old ads for Aerokits boats, the 46" Crash Tender was sold as being suitable for 2.5-5.00 cc engines, whereas the Sea Queen, same length but 1 1/2" more beam was advertised as being for up to .60 glow - though I saw plenty back in the day with larger engines, including petrol of at least 20cc.