The 16hp Onan is HUGE.
These pictures are from the ad:

The yellow "shell" is a wooden block holding the hood off the battery.
My son took one look at it and declared it as his.
A few things to work out:
On idle the engine surges or hunts. Reading, it sounds like an air leak after the carb.
Need to investigate a ticking sound.
There's a mouse nest beside the carb. - see if runs better without it.
Eventually, I want to find a mower and set this up to cut grass.