It is quite likely that the .20 will fly the model satisfactorily but it is always a much safer bet when choosing an engine for a model to stay close to or at the upper limit of the power options rather than on the low end. Since you already have the .20 mounted go ahead and fly it as is, as you might be pleasantly surprised as a present day OS Max .20 is about as powerful as most .35's of old, when that kit was designed.

Karol