Headgasket, so you have to bore the engine??? Doesn't make sense.
He means rebuild the engine and bore it out or???
My opinion IF the motor needs to be rebuilt..
Grab a R32 RB25DE, throw a thicker headgasket in there and drop the motor in, bolt up all your RB20DET stuff and it will run.
Literally, the ONLY!!!!!! bolt on part you will need from the rb25de is the inlet manifold, use the rb20det inlet plenum (that sits on the inlet manifold).
Get the ecu remapped or drop a PowerFC in there and enjoy more torque and more power with less boost.
That will probably be the cheapest way around it.
If you go the R33 RB25DET it will cost more outright and you will have to spend money on wiring mods to get the rb25det up and running.