Folks should we not step back and breathe a bit here. I'm no sub expert but this apparently isn't going to submerge either. It looks like the original question is valid, but there's plenty of info out there, and modifying plastic cits is no secret in the magazines and the web. But looking for an exact answer and potentially "complaining" nobody's got an exact answer isn't a problem, neither is looking at the afforementioned picture and figuring out the original plastic shafts couldn't be replaced with a bit of brass tube and rod, some epoxy and filler etc. Few pounds worth if not salvegble from something else. Thats what modelling is about, there's probably dozens of folk just get on with it so there's no need to publish an exact step by step.
...sorry, that's not meant to be a dig at anyone, bit it doesn't look an insurmountable problem, unless you specifically want to wait for a specific prescribed step by step. This is why marine modellers are so good in my oppinion compared to railway modellers -we learn how to make things work without spending hundreds.
...As for tank steering why not give it a try in the bathtub? It works for lots of models and toys with props right next to eh other. If I want to try something like that or a pump for a bowthruster rather than buying an expensive item I fill the bath and have a try with something jerry-rigged first. It costs time rather than money, and you learn from it.