The basics of 2 stroke tuned exhausts are explained here:
http://en.wikipedia.org/wiki/Expansion_chamber

It would be possible to tune a Victa exhaust system but the system would be very bulky. Essentially you would probably begin by finding a motorcycle system that had the same pipe diameter as the Victa exhaust port, then tune the length of the pipe upstream of the expansion box to give boost at the chosen speed. Victa engines are limited by their mechanical strength to quite low speeds, so this section of pipe would need to be much longer than on a motorcycle. (You would want the system to operate from about 3,500 rpm). The whole system needs to be as nearly straight-line as possible: any bends must be very large-radius curves.