On my Wing mfg. plane the 91 was overpowered. It flew fast and had gobs of power. I used a Graupner 14/7 2 blade prop and it zinged along well. This plane is bigger and heavier. Usually WW II aircraft tend to come out tail heavy. especially round engine variants. If I had a 120 I would use it. This plane will have excess power with the 91 but not overpowered. I fly at 600 feet. If you are at a higher altitudethen definately go with the bigger engine. Make sure that all of your mods do not add any tail weight to this plane. Adding ballast weight is bad form.
I have made plates for the 615 gear and they should drop right in.
Greg