I personally like to dismantle cheap RC cars for my conversions, even though sometimes they tend to break easily...

Also like to install my electronics+motors into separate cylinders, they kinda act like removable modules you can install into other RC-submarine hulls that fit the WTCs. This has an another added bonus in case of flooding as only one compartment could be flooded instead of the whole cylinder.

As for motor mounts, I usually make my own also (why spents $$ on something that you can scratchbuild straight from scrap styrene laying around at home?) as it lets me play around with gear arrangements in order for them to fit inside the cylinder.