The fill and drain plugs have the same thread. Motorcycle stores often stock our size.
If you want to put a magnetic plug in the drain hole, make double sure the plug isn't too long! If it were, the ferrite magnet would touch the outer wheel of the differential and break into bits and pieces and they be magnetic and stick all over the steel inside parts of the gearbox! Ferrite is very brittle!
I installed a magnetic fill plug for that reason, and it does collect a little bit.
Btw: These gearboxes have a little magnet built inside, not normally replaceable.
This one is pricy and there are cheaper versions but I like the magnet strength and design. For $25 I will probably just use it in the drain plug as I imagine most metal makes it to the bottom when parked. If i decide to put one in the fill plug that is easy to change from the top.
View my fuel log 2018 Mirage GT 1.2 automatic: 37.9 mpg (US) ... 16.1 km/L ... 6.2 L/100 km ... 45.5 mpg (Imp)
Changed my gear oil with Red Line thanks to this thread.