Easiest done on a BMW Z3 by removing the whole front valance. This will give you the access you need to prepare the car for conversion.

Jack up the front of the car so that you can easily get in underneath

Remove the undertrays and any screws holding the wheel arch linings to the bumper. It may be worth removing the front wheel arch linings at this point so you can inspect the front of the sills.

Disconnect the sidelights.

Undo the two vertical bolts holding the bumper on through the circular holes in the bottom. You will need a large star bit.

Remove the bumper

Open the bonnet.

Disconnect the wiring loom from the left side first, removing the 10mm nut to release the ground wire, disconnecting the washer pump, the left light cluster (multiple connectors), the horn, and then moving over to the right horn and headlight. Disconnect all the clips that hold this loom in place and secure it out of the way somewhere on the right. May be worth wrapping in a bin bag if it is going to be off for a long time.

Remove the headlights. There should be four hex head screws (8mm?). You will need a long extension for the vertical screws on the left and right hand corners.

Remove the bumper shocks

Centre punch and drill out the spot welds holding the front valance in place, underneath the bumper shocks

Undo the screws holding the bonnet release mechanism to the valance and feed the catches and the cable out. Secure these in the engine bay.

Disconnect the clamps that hold the radiator in place

Remove the 10mm hex head screws that now retain the front valance and remove it. Be careful to secure the threaded clips into which these screws seat.

You can now begin dismantling and removing the engine. Start with draining the fluids and disconnecting all the ancillaries. Disconnect the wiring loom and secure it out of the way.