yes, when an engine gets old the gaps between the cylinders and the pistons gets to the point where you start to leak oil through it and that gets burned in combustion
there is no way to fix it except replacing the block at least, which is the engine really so yeah
it's rare that engines still run ok at this high KM, the proton satria's engine, for example, a car i have personally driven, is notorious for doing comfortably up to 500000km but this is the top end of modern internal combustion piston engines in production (yes, it was a very popular taxi for a long time, and personally i liked driving that car)
so, yeah, if you can get a fresh new engine dropped into it, it will run another 250k at least, if the rest of the body is solid, the frame the skin the glass the electrics, it's mad to not just get a new core installed, IMO, if it did that well
i presume you are saying 300k MILES so that is really fuckin impressive if so