The Yerf-Dog will not take a sprocket larger than 39t without moving the axle or engine. Even with the 39t sprocket, you will need to grind down an unused boss on the engine case for clearance. I ran the 13t drive sprocket with 39t axle sprocket on mine. The Yerf-Dog already has high ratio gears in the final transmission, so that helps it along too.
I ran the A12 cam with a Hammerhead exhaust and UNI cold air intake. I did not do any cylinder, crank or head work, but that will also yield more grunt.
Ignition upgrades will let it spin freer at high RPM, but did nothing for my low end.
Tires make a big difference. I ran 22X8-10 tires on 10x8 rims up front and 22x11-8 tires on 8x10 rims in the rear. running tall tires up front allows the front to roll over things instead of hang on them. Shorter rims in back allow you to air down more without rim damage which gives you better traction than a tire with less sidewall.
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