A fellow in MO is selling new gauges on e-Bay that I think will suit you.
The M5 diesel and gas had different thermostat/upper water manifolds. I suspect that the 602 was the same. Welters 417-498-6496, I would think can get you what you need for thermostat and water pump parts.
You'll need a press to get the pump apart which can be risky (breaking parts). Of course it's not near as stressful if you have new parts in your possession.