Miraculously the bleed screw is gone - nobody knows a thing.
Where can i get a replacement or can i use a similar threaded 'bolt' - is there a seal involved somewhere?
Obviously an original would be best.
A bleed screw is mostly just a screw / bolt and a ball bearing. The screw / bolt pushes the ball bearing onto the seat of the pump body and seals the bleed off passage off. So yes, if you can find a thumbscrew with the same threads it will work. Just remember the ball bearing, not too small, not too big.
Hope it helps.