Leon,

has your project flown at all? If not, you might want to consider reducing
the variables somewhat by either purchasing an Onan firewall forward
package( they are not exactly a sought after commodity) and using that to
debug the airframe, learn to fly it and then or parallel to that effort
develop the new power plant.

If the airframe is a proven performer, have a look at one of the back
issues of "Contact" which describes Leon Davis's conversion of the engine
to which you refer - it will save you a great deal of grief. ( he is a
pretty smart engineer and did nice work on that project)

A final comment on low HP powerplants. a 2 hp degradation in performance
is not much normally, but if the engine is only good for 20 hp it is a 10%
reduction so maintaining optimum tune is extremely important. Higher HP
engines are a bit more tolerant of being slightly off song. Performance
is closely related to power to wieght ratios. If you want the best from the
power available, get the weight down and keep it down. Too often the
decision is made to put a more powerful engine in to compensate and the
attention to wieght saving is relaxed. The net result being that much of
the anticipated gain is lost.

The number of times I have been outclimbed by an otherwise identical Q1
flown by a pilot 20 kg lighter than I ( the fact that he was a much better
pilot had absolutely nothing to do with it ;-)) are too numerous to
mention. That said, the life of an anorexic midget is not for everyone....

John