Here is the plate on the generator motor,
 

I'm looking at the parts book for 2B1-up and it's for a D330C engine Form #UEG0015S. This is an early scroll type injection pump. Check/compare water pumps as the most obvious difference between the D330C and 3304.
Also compare the injection pumps.

That is the blown motor in the excavator

I don't have a parts book that covers that high a s/n 48Vxxxx but for sure it;s going to have a sleeve metering fuel injection pump.

So swapping the governor over is a no go. I haven't bought either one yet so its not big deal. I'm trying to see if its worth paying 5500$ for an excavator with a blown motor. It works other than the motor.

then what's a rebuild worth. you could still be getting a cheap 20 tonne excavator. depending what is wrong with the engine. blown motor is a very vague term.
has it done a head gasket or has it done bearings or thrown a leg out the side.

So I assume you are the same nocolasktu that has asked the same question over on Heavy Equipment Forums with the legendary Nige pointing out that neither the excavator nor the genset have the original engine in them. Not sure how to proceed here except as tk88 points out, what is actually wrong with the engine? Can it be repaired?
Other than that a visual inspection of the two engines focusing on the injector pumps and intake and exhaust. Nige says it should not be turbo charged so that may help with the injector clearance around intakes and exhaust piping. Everything else should be able to be swapped over.
TK88 the excavator is a 16.5 tonne machine not a 20 tonne

The engine is shot, connecting rod cap came off and blew out the side of the motor by the bell housing. Block is toast, so are the rods, piston and possibly the crank and cam. I can get a new block and switch everything over which is probably the safest bet. Or LS swap it lol

Those engines may not be that far off. I didn't catch the last two digits on the first read pass. Turns out 2B1-4363 is a D330C and 2B4364-up is a 3304. So at 2B10011 those engines would both have sleeve metering injection pumps and would have similar governors although they would be set up differently.