RE: Surestart upgrades?
I did some simple mods a while back that involved allowing the engine to breathe better, like removing the intake screen and tapering the intake port from having square edges to a bell shape. Along with PT's suggestion I might also mention balancing the prop and I have personally gotten more whoopies out of APC brand than others. I also remove the NV seal and spring and replace them with a piece of fuel tubing to limit vibration and air leaks giving more consistant runs and settings. Of course setting the piston ball helps a bunch too and is nearly required on the latest sets of motors. Experimenting with shims and fuel mixes can yield good results too. I got some running 24K static this way.
Other mods that are not so easy are cutting the exhaust slits for sub piston induction, and grinding/balancing the crank. When done right, they seem to help, when done wrong they can make the engine a nice paper weight and guys disagree about them. Stick to the easy stuff and you'll be pleased.