I have an OS 61 FX mounted nearly vertical on my Kadet Senior. It is set up such that the stock muffler exits in the center of the firewall on the bottom. It necessitated creating a tunnel at the bottom of the firewall. This "should" eliminate flooding the glow plug or hydralocking the cylinder as the exhaust port is pointed nearly straight down (I hope ). I also had to ensure that the tank's centerline was lowered according.

Good luck on your choice.

Now, if I can only figure out how fabricate a balsa cowl similar to some of those presented in various RCU threads ...