There are various ways to do this, but I think you would find the easiest way is to install a Water Tight Compartment (WTC) inside the hull. These are usually made of perspex tubes with two sealed end hatches.
Look for data on making WTCs and sealing the output shafts. Here's an example, but there are many more **LINK**
Ignore data on incorporating a dive mechanism. The Revell boat would take quite a thin WTC. Then just put your radio and motors into the WTC, and add ballast or foam to get the sub to float at the right level…