Hi Ivor, I'm far from an expert on the small gas engines, but there was a power unit based on the 15 engine used on Caterpillar combines. The accessorie drive is on the opposite side of the engine compared to tractor engines, they also have a wacky fan that is special to combine engines.
My understanding is that there was an adaptation of one of those engines used on a pull combine. Do not recall the details. Try a forum advanced search.