Had a very helpful chat today with a guy in a local tractor / garden tractor parts place, they repair them and carry various parts, so he knows from experience what he's talking about. Westwood are well regarded as one of the sturdiest and most durable.
Firstly, the good news is loads of people round here do exactly what we want to do with them- riding them round smallholdings / farms as a 'cheap' quad bike, and they're very well suited for it.
His advice in terms of potential theft is that 90% of the time they just take the battery. Batteries have a high scrap value from the lead, they trash it and flog the metal, and its untraceable. Theft of batteries from rural locations is a massive and very common phenomenon, so taking it home is well sensible. I know a guy had a tractor in a small wood, they took his battery. Other than that, chain it to a large tree and have distinct identifying marks on it (and note the engine numbers) as proof its your machine.
Don't weld the diff. It'll put more strain on the transmission and steering and something will snap, especially if you're towing. Its got a diff for a reason and is harder to steer without it. If you need extra grip, go for offroad chevron tyres (and just suffer them churning the ground). If you go for tubeless, fill the inside with some of the green liquid anti puncture gunk and run them on low pressure, that'll give you all the grip you need and make them puncture proof. Thats what experienced locals do here; take the battery out, chain them, run chevron tubeless tyres and fill em with gunk.... And have fun.