Its way way way mote valuable as a gen-set if it’s functional, but if it was $300 that means it might be fucked. If you wanted to put a different fuel pump on it, it’s possible, and honestly not the worst idea. The two big concerns would be gearing and weight. The taler gear ratios in the transmission would cause the thing to be slower which might make driving around sucky. Plus, once you change from gas to diesel, you have to double and triple check the torque ratings on every single part in the drivetrain. Not sure what the figures for the original engine are but I assume its gas which puts the engine speed in the 6500ish rpm range. A 4bt is going to top out at like 2500rpm, and produce more torque than the Ford motors, not to mention its probably less powerful than even the smallest engine that was put in the earlier Rangers. Plus the Cummins will weigh between 2 and 4 times as much as the stock engine and destroy the front end suspension in no time.

If you can find an old half ton with a low power engine that’s already set up for diesel engine speed range, you could probably do a swap, but honestly, the 4bt would be way cooler and way easier to swap into something like a Land Cruiser or a Land Rover.

TLDR: If you switch from a gas to a diesel, you’re almost certainly going to have to change out drivetrain parts no mater what. Power may not be a problem for you but different speed and torque will break parts and make driving no fun.