You're right that this is an Isuzu engine, it's a C201. The basic specs. for it are listed below:
Cylinders / Bore and Stroke = 4 – 83 x92
Piston Displacement = 1991cc
Firing Order = 1 – 3- 4 – 2
Compression Ratio = 20
Rating power (ps/ r.p.m) = 16/2200
This engine can be throttled, but as noted in the thread, it is set up with a governor on it that maintains an RPM for the specific load to keep the refrig. unit operating correctly. You would have to work with the governor if you wanted throttle control, but there are mechanical and electrical systems out there for that.