I do plastic magic / small boat conversion, but I don't waste time with dubious formulae.
I take a more pragmatic approach 😉
As soon as I have the hull built (and waterproofed with EzeKote inside and out if wooden) I weigh it empty, and then do a Max Load Test by plonking it in the Domestic Test Tank Facility (🙄) and loading it down to the Deep Load Waterline, with lead shot or just water.
Weigh again, difference Loaded - Empty = Max Load capacity which must include-
Decks, Superstructure, fittings, drive train, electronics and battery.
Then I select possible motors, shafts - home made from 2mm silver steel and brass toob, ESC, RX, battery, weigh that lot and see what's left over for the decks and superstructure etc. If not enough usually the battery is the first to be reduced! BUILD LIGHT😉
Hope this helps.
BTW: If you fit two brushless in a hull that size you better give it wings and elevators as well!!
Two little (~180 size) should be more than enough.
Cheers, Doug 😎
PS If you're worried about stability; instead of a 'block' battery pack you can use individual cells (1.2V NiMh or 3.7V LiPo as appropriate for your electronics and motors) and lay them along the centre line at the bottom of the hull.
For the smallest of my 'Plastic Magics' and Mini boats I use Micro/Nano Rx, ESC and rudder servo and check that they can all work reliably from a single 3.7V LiPo cell.
There are also a lot of pretty powerful mini motors out there that run on 3V 😉
X-Track for example.