Well, when you say it fast it sounds easy

I'd remove the complete lump and 'box from your mk2, swap the flywheel and clutch onto the abf, refit the 'box then put it back complete. You've got to remove the mastercylinder/servo too to get the wiring out and fit the mk3 wiring. Also got to run a wire to the mk3 coil to run the rev counter, or wire in the tci unit and mk2 coil instead. Then if you want to fit the ecu in the standard mk2 location you've got to extend the loom to the ecu - 32 wires = 64 solder joints (not fun btw), but does mean you can fit the mk2 raintray rubber sleeve.
Also think about the induction side, need an airbox from Seat ibiza/toledo if you want it to look standard, or get and induction cone.