The gearbox is a Ford Type 9. There are a couple of options here, mainly grouped into ‘long’ input shaft ‘boxes and ‘short’ input shaft ‘boxes. In the context of the Midget, the important differences are these:
1) Long and short input shaft ‘boxes need different bellhousings
2) Some ‘boxes are closer ratio that better match the Midget’s original ratios
3) Short input shaft ‘boxes are generally rated lower bhp (140) than long input shaft ones (200)
Short input shaft ‘boxes some from Ford sierra (1600 and 1800) and are more common than the long input shaft versions, which came in the Capri v6 and also in diesel transit vans (among other things). The diesel ‘box should be avoided as it has a very low first gear and may have had a harder life hauling a van around.
The Type 9 gearbox is heavy, due to a cast iron casing, and if you are rich then alloy casings can be got from Flowtech or BGH gearboxes.
It is worth noting that the bolts that join the bellhousing to the gearbox (tapped into the ‘box) are an odd size, 12mm x 1.5 rather than the more normal 12 x 1.75, I had to order some from a hardware shop.
Both short and long boxes need the input shaft shortening so that it doesn’t hit the tail of the crank; I cut 15mm of my long input shaft box.
This was done with an angle grinder while the input shaft was turned using the output shaft with the ‘box in gear.
Long input shaft gearboxes need a different bellhousing from the short input shaft ‘boxes. The long shaft ‘boxes can use a Caterham bellhousing, the short ones can’t. Frontline developments can make either sort depending on the gearbox required, as can Titan (who also make rather nice roller rockers for the A series). JLH Morris Minor also retail a K-series kit, although I’m not sure which length input shaft gearbox is designed to fit.
Apart from the length of the bellhousing to accommodate the different shafts, the Caterham bellhousing has the clutch arm slot on the top rather than on the side, where the Frontline one is. This might make a difference to the way that the clutch cable is routed and also to how much needs cutting out of the car to get the engine and gearbox in.
It is worth noting that some gasket sealant or a full gasket needs to go between the bellhousing and the gearbox. There is a hole in the front of the gearbox that the selector mechanism pokes through, this needs to be sealed so that oil doesn’t come out.