Are there any free or cheap power mods to do for the SC400? Like any coolant bypasses (away from the throttle body) or how to do any PCV mods.

 

You can open up the air intake down behind the front bumper,just trim away a couple of inches on the bottom of the duct from the opening rearwards.
A K&N filter is a good idea for the $,but it's not free.
The airflow meter has a dividing vane between the metering passage and the bypass area that's about an eighth of an inch thick and flat on the leading edge;you can file a more rounded edge on this portion to increase the airflow,then remove the plug hiding the idle mixture screw,and dial that in to a more precise setting.
That adjustment was done by running the adjuster in/out,testing by driving,then when your adjustments become less than a screwdriver blade's thickness,you're there.It took a couple of days to get it right,but it's worth it.
The pcv is a good idea;I'd leave it alone.
I replaced my plugs/wires before attempting these things,and that made a big difference itself.

 

Do you know if the SC's top end is fuse limited like on the Supra?