The real motor might fit

The motor might fit if I make a complex adapter plate which brings the motor inside the bell housing. The gearbox input shaft is bigger than output of the motor and does not require a pilot bearing. Hopefully the input shaft can be ground smaller and a new spline cut so no coupling is required. This will allow the motor to move another 35mm into the bell housing (in the photo it's resting on the end of the input shaft).

I've got the motor on a hoist, but the pile of bricks and bits of wood isn't allowing me to precisely locate the gearbox. There is some scope to raise the whole gearbox and also to rotate it around it's output shafts. I'm hoping that this, combined with moving the motor further into the bell housing, will be enough to allow the other end of the motor to clear the subframe. I'll hang the gearbox from the ceiling and then I should have better luck.