Are we talking about the diff outdrives? I didn't realise that they were ever plastic but then I've never set eyes on a Minion.
It would appear to me that you need to look at the TR04-Pro for your upgrade parts.
That also provides your slipper clutch parts and the upgraded transmission. It looks as though the long plastic cups from the slipper/spur can be replaced with T04021 in steel. But the prop shafts themselves are replaced with ones that have CV joints in them. So I doubt that you will find a steel equivalent for the short plastic cups.
The ball diff of the minion is replaced with a gear diff, the outdrive of which look fairly standard. They have the part number TS4040.
Take a look at the TR04-Pro manual:-
www.absima.com/absima_uploads/cars/tr04pro/
TR04_
pro.pdf