Got the donor gearbox on the mower. She runs. All gears working though took a while to adjust the selector rods/linkage. It even cuts grass.
Only to find one of the pto pullies had seized and the belt worn a groove in it. Thus the flails won't turn for the grass collection end. £24+VAT plus postage for a new one. Buy new, I think not!
Went back to the scrap mowers and salvaged the same tensioner pulley albeit of all metal construction. Guessing an older variant. Rusted solid and then some. Some chipping off of the large lumps of rust then into the citric acid barrel for two days. Managed to get it all apart just now, some more chipping and scraping and back in the barrel for a final 24hr dip. It's salvageable. Bushed rather than a bearing. A bit of zinc spray and packing with grease and it'll be good to go.
You should have seen it before it went in the acid!