Hello,
Since you already have the plane, install the radio etc. Then attach a container of some sort to the firewall. Add weight to the fire wall, until it balances. Then weigh the container with the weights in it . This will tell you if you can afford to put the heavy engine in the plane without major tail weght added. The 45cc sounds good to me for a 78" span and about 1100 square inches area.

Dave