Going by their numbers of 15.5mm bore and 12mm stroke, PI()/4*1.55^2*1.2*0.061024 = .138 say .14 cubic inch displacement. Not having mic'd it may be that is more approximate and your numbers closer to actual. In this case my friend wants to extend the exhaust to get it away from the plane with about a 2 inch tubing extension through bottom of plane, so stinger is required for that.

Regarding exaggerated manufacturer displacement classification figures, my Enya .09-III TV has a bore of 0.512 inch and 0.480 stroke, which calculated out comes to 0.099 cubic inch. It is more like a .10. My OS Max .15FP-S OTOH comes out to .151 which is closer to actual.

In this case use is strictly sport use and not speed, so even with the slight rpm loss it will still do fine. One thing I did notice is that the muffler is screwed in place with 3 aluminum gasket washers. To change angle from straight down to another orientation, one removes washers until desired orientation is achieved.