mcracer,
You building the G5 for racing? Dirt or street?
I have a G5 or a pile of a G5 and am going to build it for the new AHRMA 100cc class. I have built a dozen or so Hodaka 100 and 125cc racers, a Penton or two and a couple of Yam's in the past and look forward to figuring out how to tweek a G5 motor.
I hope someone has some info on the rotary valve mods. Usually widening the port slightly is about all you can do. Which increases the amount of time the window is open so more air flows, but it only works up to a point. Usually the height of the window is fixed and shouldnt be messed with.
Looking at the valve timing specs of the G5 shows that the port opens 5 degrees later and closes 5 degrees sooner than the F5, so I am guessing that opening up the window by about 5 degrees on each side will help.
Then cleaning up (hand porting) the intake and exhaust ports to streamline and improve flow.
Also, the carbs are 19mm (I think) and going to a 24mm (from the G31M or F6) or so would be the trick IF the airflow can be increased. The trick will be to see if a larger diameter short bodied Mikuni can be fit to the motor. But with the larger carb you have to watch the air velocity in the carb doesnt get too slow with a bigger diameter carb or it gets sluggish on the botton end, and tends to load up when you come into a corner and chop the throttle quickly.
In answer to your question about a pipe, I do have a Torque Engineering pipe for the G5 that I am going to use and I see there was a G5 aftermarket pipe on ebay earlier today.
Anybody else with any ideas on hot rodding a G5 or a rotary motor I am all ears!:-D