Welcome....
Doing a bore and oversize isn't going to make summer out of winter on a TD42. With a 22 (odd) :1 compression ratio, all you need is some quality rings on quality pistons with the machining work done by someone who knows what they are doing, especially if you are fitting new sleeves. The sleeves MUST either protrude slightly ( can't recall off hand by how much, or, second choice, be flush with the block.
When I rebuilt mine about 5 years back, I fitted alfin type rings and pistons ( designed for turbo diesels ). Basically the more compression the more the rings squeeze against the sleeves, or the better they seal against the sleeves. A good quality head gasket of the correct thickness, and new head bolts and that motor will be running like a dream.
I'm in South Africa so your budget is difficult to work on from my side, but a decent turbo with intercooler and an little work on the injector pump with a 3" exhaust and you should be good to go. It all depends on what power you want out of the Patrol. The Americans have a saying... " You can make a brick fly.... if you put enough power and money behind it"