The first one is a 1976-1978 MKIII Mustang, using the Series 70 MKIV engine, and the G4 carburetor (The carby you have fitted is actually an LM, but it will do Just fine)
The second is a little older, a 1974-1976 VC-Mustang MKIIA, this one appears to have the original LM, and also uses the Series 70 MKIV engine.
This engine was quite a solid and reliable engine, and given the TLC these mowers are getting, you should be able to get many many more years of reliable service.
Be make sure when looking for parts to get parts for a VC-mustang, not a VC-160, as the VC-160 ran a different carby and engine, and used a totally different chassis and catcher (Not trying to knit-pick here, just don't want to see you run into any troubles)
The catcher I might be able to help with, I should have a trip to Sydney coming up soon, so I will scout out the catchers at the local tip shop, and if I find one, I'm more than happy to grab it for you, the shop charges $5 - $10 for catchers, however, if I happen to be grabbing a mower at the same time, and it does not have a catcher, I'll pop one with it, as that way it's free.
I'll let you know what I find mate!!!
