The spare dif is the Victa VC-160 Autodrive MKI Gearbox, this was the one with the full metal drive control.
The mustang engine will not fit, the self propelled engines use a longer crank shaft to accomodate the pulley above the blades.
I do have a mower that would help you, its the 1980's side-starter version of yours, it appears somone had issues with the drive, so the belt has been disconnected, and the controll removed, I will make sure the original engine is on it (long shaft) but if so, would you like it (it has spark & Compression, but i have not tried to start it yet)
it is a power plus, it does have the dif still in, and it has many interchangable parts with yours, enough that you would have a complete mower between the 2. if you want it, we can work out a price (I'm more for trading than cash if that helps, and i'm not after much for it) and i could meet you 1/2 way, say Aulbry? (Not sure if you like driving as much as i do, but that would make a nice day trip for me)
I'll have a look at the one i have tonight, and let you know if i think it will help you, if its in the condition i think it is, you will need to transfer your mowers dif (not the spare)wheels, control lever and belt, onto this one, then you should be good to go.
