Honestly, I would not even think of a gas engine on a 60 sized plane. Just getting it in one could be a problem let alone getting it to balance not to mention the weight penalty would kill the flying qualities but if you must then about your best choice is the Zenoah G-20.

Of course you did not mention what plane you are considering doing this with. A huge Cub, Telemaster or something along those lines might just work.

Just to give you an idea, I had a G26 that was coverted to electronic ignition that I stuffed into a 120 size Super Stearman. I used it because the recommended glow engine also required you to build a weight box and stuff 18 ounces of lead in it so why not use something that is not dead weight and the G26 fit the bill nicely.

I would think on most 60 sized planes, the G20 is gonna be 1/4 to 1/3 the total weight of the plane