It should still run even if your lines are backwards, you'd just build up a lot of pressure in the tank. I'd done this on my stock tank and when I'd remove the gas cap it would blast out air and the vent would be leaking gas, but it ran fine. Stupid question but have you removed the plugs and...
The only thing that would be difficult to source that might go bad is the coilpacks. S15's have the ignitor built into the coilpack (thats why S15's dont have ignitors).
S15 has different coil packs, no ignitor, inconel exhaust divider flange on exhaust side, dual mass setup, different ECU/Harness, and the very shitty AZ6 transmission. Thats it
EDIT: Bigger injectors aswell (480)