The next "cheap" mod you can get would be a set of reeds, but that's about as far as you can go with cheap, bolt-on mods.
I would suggest suspension mods next, but we'll keep the topic limited to the motor. The next motor mod would be a complete porting and head modification. Most any big-name shop that does porting will also modify your stock head for higher compression, and they usually have a package deal. You can also go with an aftermarket head, which will give you interchangable domes and more versatility, but also adds to the price.
If you get porting done, be sure to get it matched to your existing pipe, or replace your pipe to match your porting.
Duncan Racing 340PV, +2 A-arms, +4 LSR axle, and a host of other goodies.