I went for a second hand HDJ 100 unit off eBay, turned up in crap condition, got a refund and got to keep the unit. Stripped and cleaned, fitted new plunger/contacts and all was well. I then modded the front cover and fitted my 80 24v dog and fitted it so the 12v mod was complete. One minor issue was I got abut carried away lubing the plunger and the grease migrated up to the contacts leaving me with no starter the first rainy night in Spain for bloody months! A quick push and got home and then cleaned the contacts and all was well, total conversion was about 30 quid for OE plunger kit IIRC?
As yours is a manual Frank you might to try the second hand route?
regards
Dave