In 2018 I tore down Bonnie's gearbox because I found quite a bit of bronze 'sludge' in the outer cover.
After removing the gear cluster I found the problem: the drive side (DS) layshaft needle bearing thrust washer.
Its locating peg hadn't been sufficiently proud from the case to retain the thrust washer from dislodging and turning. The thrust washer quickly jammed itself back against the peg, damaging it. The peg ended up gouging a groove in the washer's soft bronze, accounting for the bronze particles I found in the gearbox oil.
I'd had a proper drift fabricated - one with a groove in its shoulder for the locating peg - but I'd still managed to drive down the pin while replacing the layshaft needle bearing in 2016. This is what finally convinced me to get eyeglasses.
After a whole lot of sleep loss, I took the entire engine unit to Bob St-Cyr, an exceptionally skilled and talented machinist, so said by many.
Bob fashioned a jig which he used to drill another hole and install a new peg after grinding off the old one. The jig is installed from the inside of the gearbox and then fastened in place with an Allen screw and washer tightened against the outside of the gearbox.
The jig installed in the gearbox and tightened in place from the other side. Hole for the new locating peg is at 11 o'clock.
Later, after several unsuccessful attempts at installing the layshaft and mainshaft bearings, I returned to Bob's shop with the engine and he pressed in the bearings in no time at all with a 50-tonne industrial hydraulic press. To facilitate the operation I made this engine base.