Try this for an engine kit in CA. Custom Motorized Bicycles / Bicycle Engine Kits / Los Angeles| VeniceMotorBikes.com
As for your problem there could be several things causing it. 2 cycle fuel, once mixed has a relatively short 'shelf life' A few weeks and it begins to go bad. How old is your fuel?
Have to looked at your spark plug? What color is the tip (electrodes and porcelain) It might be fouled. You'll want a nice dark chocolate color. If it's black and oily clean or replace it.
What fuel mix ratio are you using. With the milage you've reported it is well broken in so you should be doing something in the 32:1 range.
What has happened to the previous magnetos? Did they just quit or get wet? Is there any sign of oily residue inside the magneto cover?
There's several things to think about. Get back to us with what you find or have done to check this loss of power you're experiencing.