Looks like OS, Saito, Valley View, RCGF, and NGH (new to me) have options from 9 - 15cc's. When Evo was still in existence, I believe they were producing a 10 cc and 15 cc size gas engine.
Ten years ago, it seemed like anyone with gas engines smaller than 20 cc's were not content, so glow power was determined to be the best option for this size.
.61 is 10 cc's; practical gas engine options begin at 20 cc's based on what I've seen flying at the field.