I don't know what all is available as far as mods for the Moto4's, but you may be able to find at least an aftermarket slip on silencer, a Uni or K&N air filter, and jets for the carb to rejet it once you get more air flowing. If you can find an aftermarket camshaft to go with the silencer and better flowing air filter, you should really notice a power improvement. I don't know a whole lot about the Moto4, does Yamaha still use the same basic engine in a current quad, like the Raptor80 or something? If so you may have a lot more options to choose from. Depending on the type of riding you do, you might want to think about changing tire sizes if it's shaft drive, since that's the only way to adjust final drive gearing, shorter tires would give better acceleration and better handling on tight corners at speed, taller tires would give you more top speed, and a more comfortable ride at slow casual trail speeds, with increased ground clearance. IF it's chain drive, you could swap the front sprocket to help power or speed. If you want more power for climbing hills, and quicker acceleration, get a front sprocket that's 1 tooth smaller than you have now, if you want more top speed, get one with 1 more tooth on it.
Even if you want to stick with the stock tire sizes, a firmer tire would give you better handling, if the tires you have on it now are the bouncy 22x11x8 balloon tires that have the soft sidewalls that roll over in the corners easily (fairly common on older quads and 3 wheelers) a set of 22x11x8 Kenda Vipers would really help the handling, without breaking the bank, and also give great traction in any terrain, while lasting very long. I have a set on my 200x that are now over 10 years old that are still in reasonably great shape, no weather checking, usable tread left, and only 1 flat in the 10 years I have had them (even with being parked outside in the yard year round with extreme winters, and ridden very hard in rocks mud sand snow and lava cinder roads). I have 2 friends that just had me order 2 sets for them for their 3 wheelers after they borrowed my old set for this winter, and had them out due their 1 1'2 year old tires lol. the 22x11x8's were only $43 each without wheels thru Recreation Tires
Hope this helps, Good Luck [img]images/graemlins/40BEER.gif[/img]