Rao,
The TD42 and TD27 incorporate the same parts, the TD27 is 4 cylinder and the TD42 is 6 cylinder, piston diameter and bore size is same for the both of them. That is 96 X 96 mm.
Finding parts is very easy, go to Abid Autos at Montgemery Road, and you will find everything you need, parts are readily available! my GQ is a 91, and has had 5 rebuild on it! 3 of which i got since my two years of driving! The problem is finding the engine it self. A simple answer to your question is: parts are readily available in the market, but pricey.
First of all if you keep your engine well taken care of it won't die on you. Think that a Patrol is a big mean truck which can take you anywhere at nominal speeds, and not a honda which takes you at blazing speeds on the road! people have this idea that with a engine this big the vehicle would be able to do tremendous speeds, infact a TD powered patrol can climb a 100 degree hill without any hesitation. If it dies and you want to a replacement rather than a rebuild, go for Toyota LC engines of the same decade, they fit in very nice. As for gearboxes of they never die, i've got 500,000 kms on mine and its still good as new. If you go for another engine, the gearbox can be coupled with the existing one, just some mods and you will be good to go.
As you say that at higher RPM you loose power, that is due to the fact that the fuel injector pump is out of order, when the diapraghm has expired, the pump does not funtion properly, and you have power loss, very loud voices, shortened gear ratios. There is a trick to that, if you happen to ever try that vehicle again, push the choke lever on the pump towards the radiator and let it come back to its orignal position and you will be able to feel a huge amount of difference in power when you drive it. But this is temporary, this means you need to rebuild your Fuel Pump (source: personal experience)
Over all i'd say if you are person who is into off-roading and likes big mean vehicles, then you are good for it, with a TD42, but you have to be caring!
if you want some info on Patrols and TD series contact me at sherwan_88@hotmail.com
Sherwan
