Mine has a Saito 1.00 and if it had any thing smaller , I would be disappointed . It will hang on the prop with this engine.
If I had to go with a 2 stroke , the Supertigre .90 with pitts muffler would be my choice . Also don`t be afraid of Magnum or ASP 4 strokes . I own a .Magnum 4c .52 an a ASP 4c 1.20 , both are great engines.
If you can hang on to the O.S .70 , you`ll find it a perfect match for most .40 size planes. Which there are many.
MinnFlyer and I were at a Funfly a few weeks ago, where a guy had a Skybolt with a Saito 1.25 . It was pretty impressive ,but at the upper limits for the airframe.
One of the things I like most about the 4c, is clean up . It takes very little effort to keep the Skybolt clean.