I think the bc4 you refer to is the gearbox, assuming that is an old 4 speed unit then you are likely to be quite restricted to what will bolt up to it....I wonder if the Cvh engine from then MK 2 XR 2 will fit and that would offer up to 2.1 quite easily plus you can get the zetec head on there and even turbo it but the max torque that box will take is likely to be the issue as it wasn't designed to take 3, 4 or 5 times the power.