GM, which I am familiar-ish with, has had several series of
PCMs in the space of a decade.
Mass market vechicle mfrs have been encrypting the PCM
access and third party developers have learned to break
the encryption enabling consumer tuning cable/software
products.
Smaller marques with limited-run controllers don't get
the same support, tuning tools are thousands rather
then hundreds of dollars.
A question is, whether the generator ECU is the same
as might be found on Cummins diesel trucks. If so
then you might fins an existing solution, or find the
vendor of existing solutions capable of, maybe even
interested in gen'ing up a specifically compatible
code for you. I know the outfit who made my tuning
package, will accommodate folks who have low-run
vehicles if they use a "standard" PCM (code is platform
dependent but the same GM PCM hardware may serve
20+ model*year combos).
A key is, does the generator have an OBD-II interface?
A serial link of any kind? If no then all you have to go
on is old school - air, fuel, spark - with the instruments
you have at hand.