Jimny cooling systems are pretty stout, a car that young shouldn't need anything changed. In any case, you can buy fully welded alloy radiators with a thicker core from eBay. The stock radiator is plastic tanks but it's pretty well protected and easy to put a bashplate under. The thermo on the front is just for A/C, they run a viscous hub fan off the motor for the radiator.
The airbag wiring is easy to find as it is has yellow conduit. I don't know what happens if you disconnect it, presumably you get the airbag light. In any case, there's big section in the manual that states all the accidents the airbags wont go off in. Basically they wont go off unless you really, really need them. Certainly they wont go off with general offroad hooning.
Tyre size depends on what you're doing/where you're going. VVT Jimnys have (relatively) good low range gearing so I reckon _for a hunting vehicle_ you could drive pretty happily and be reliable on 31s. If you're going to be doing technical 4wding or big hills or being particularly abusive I'd probably stick to something like a 235/75r15 (29") to maintain reliability and gearing, you can fit these on the stock rims and they will be a lot easier to fit into the wheel arches too.
I have no idea where you are but here we have lots of blacksoil type country and not much in the way of technical obstacles (to load up the drivetrain) so I'd run a 31 or 33 for minimum ground pressure.
You haven't said if it's auto or manual but if it's manual you can fit the transfer case from a VVT auto model to reduce gearing by 32% in both high and low range. This is a straightforward and affordable way to get some lower gearing.
For your axle I'm pretty sure non ABS axles are the same but you'd want to confirm that. The diff housing may well be bent too.
Jimny heaters are fantastic, you'l enjoy that.
