Keep it original - options are to overhaul the engine, which is, as suggested, difficult and expensive, as parts are hard to find and expensive. But it may still be possible to find a good engine from a rusted out chassis - there were a lot of military ones sold to the public, and there should be a few of these that have succumbed to rust rather than engine issues. (I still regret not buying a new one of these engines that went for $200 at a clearing sale about fifteen years ago....But I didn't have or want a six and my shed was already cluttered!)

Alternatives - the easiest conversion is to a Holden engine, and more grunt is readily available, but this means a big drop in the collectible value in the future - all depends on what you think of it as - a collectible or a working vehicle. Other conversions, particularly diesels may be better, but will reduce collectible value more, and probably cost more.

This vehicle, being a long wheel base, has a salisbury rear axle, and you will not break axles in this, regardless of what engine you put in it.

Note that Holden conversions have a reputation for breaking gearboxes, but this is due to inaccurate alignment of the adapter rather than the increased power. The Rover gearbox will stand up to around 120hp, more if treated sensibly, although the early all synchro box is not the strongest one made (best is the last of the synchro boxes or late 2a box.) The transfer case, after the increase in the diameter of the intermediate shaft in early 2a is just about bulletproof.