So scanning through the rules it appears STU is limited to 3L and six cylinders. However, I see turbocharging is allowed and that engine swaps are allowed, i.e., Acura in a Honda, or one Ford motor into another Ford.

Based on that it looks like it'd be possible to use a say 1998 Mustang (which meets the year requirements) and then put a 2.3L turbocharged Ford Mustang engine (OEM in the SVO Mustang and drops right in) into the chassis, run a 37mm restrictor and have a spec weight of 2770 lbs.

Question from a curiosity standpoint - why is turbocharging allowed but supercharging is disallowed?