Not really much for cheap if you want billet aftermarket, if you can access machine shop, any less rotating mass = better performance, whether it be drive line or engine. Honestly any stock engine with a better flowing intake and a decent carb or set of carbs and a less restrictive exhaust, along with lighter flywheel (not saying mill/turn a bunch of material off yours, rather, buy a Zach Kerber or ARC) and a high output ignition, more ignition timing and slower burning fuel, as well as a head cleaning, and head milling for more compression (without P/V interference) can usually wake up an engine rather well, and stock internals will usually hold up to 4500-5000 RPM. The stock cam is designed for peak torque and HP at 3500-4000 on most modern engines, so a slightly more aggressive camshaft may be an upgrade. Look at it like a car engine of any type, there are certain limitations you need stay within and as long as you do that you should be able to make a it little more powerful all around but most of all more responsive which is the key to performance engine. I personally focus on chassis a lot more than engine. Lower HP, Better tires, more responsive and happier chassis will usually outdo, big HP with decent tires and a chassis that looks good on paper.