When my XR600 became harder and harder to start it was down to head bolts stripping threads and pulling out of the cylinder. I've had to helicoil all of them. The lack of compression is what stopped the bike starting.

I've been told that overheating the engine changes the metallurgical properties of the alloy, softening it, and that's what allows the bolts to pull out. I haven't got the equipment to check for softening so it may or may not be true but I know it does happen in overheated car engines.

As the XR's air cooled it's very hard to know whether you've overheated it or not. If your days consist of roosting deep sand in 1st gear when it's around 40C then you might well have pushed it too far but I never had any issues with normal trail / road use in UK temperatures.

What was wrong with the oil pump? It's a pretty simple system and unless you've stripped the drive gear it should work ok. If it's debris that's got in there is there much on the gauze screen (just below and to the left of the pump).

The oil feed to the head comes through the outer cover before it goes up through the bolt on pipe. The weak spot (other than the pipe blocking) is the O ring seal from the pump to the cover. If that's missing or broken it'll reduce the flow and it's easy to overlook when you're putting the cover back on.

Up in the head the two bits that go first from lack of oil are where the rockers and cam rub together and the centre cam bearing. The end ones are ball bearings and survive pretty well but the centre one can look like a ploughed field if it's oil starved.

If the oil pump is junk I'd be worried about the state of the gear teeth as well as one of the pump outlets feeds the gearbox. The early XRs were marginal on gearbox life, with gear pitting a problem, so an oil starved one wouldn't be good news.