stickers!!!! lots and lots of stickers!!!! they are cheap, they dont vibrate off easily, and they wont cause serious engine damage if misaligned or worn.
it doesnt get any less expensive or more reliable than that and each sticker gives you 3-5HP if using redneck algebra to calculate gains.
now, what are you looking for when it comes to mods? more speed in the rough stuff or more HP?
HP makes you faster on smooth stuff, suspension makes you faster on rough stuff.
do you want more top end or more low end grunt?
lots of diffrent things we can start you out with. give us something more to go by please