Heres another spanner in the works......

A 60 size plane makes a pretty good home for a 15cc gasoline engine..

Theres are two currently available, JBA 15 and the RCGF 15.

No more castor to wipe off
Although many hobbyists promote castor as the holy grail of lubricants in reality there are far better synthetics for the purposes of lubricating a glow engine these days. PEople tend to cling to old habits in this hobby...
Most fuel manufacturers offer a Methanol fuel with synthetic lube and its good enough. Castor makes the engine sound smoother but thats means nothing when its gumming everything up inside and making a royal mess over the plane.

Consider Gasoline also

If I had the choice between 2stroke .61 and a .91 4 stroke, I would base my decision around airframe considerations and propellor requirements for the airframe and then consider the engine.

Wasted power often results in high fuel consumption and poor thrust. Determining engine characterisitics for a plane without  any consideration to the typ of flying the pilot wants toengage in is a shot in the dark at best.

Consider these:

Flying style:
Sport , Then 2 stroke .60 LA or .80 to 0.91 4Stroke is sufficient running mid range prop ( the if the recommendation is for 12x6-8, 13x6-8, 14 4-6, then 13x5 is fine
Aerobatic, then .75  to 1.00 ballbearing engine running a larer low pitch prop like the 14x4/5

Also bear in mind that if you opt for 4stroke you need to pay attention to vibration damping and reinforcing the entire airframe, this adds weight