If your budget is thin....Start with exhaust mods, go with deleted or highflowing cats and Y's... Go with some highflowing mufflers and dump those restrictive stock mufflers...
Go with the BFI intake design...It's free and flows great...
You can then look into ECU piggy-back and tune, headers, cams, Torque converter, etc...
Lastly look to boost, turbo, or supercharge...
If you don't know what that is all about just search for "results of mods to SC400" and check out my thread I break it down on how to modify the SC400 N/A... Good luck with the mods, be informed it will never be a speed demon unless you boost it...