[Graham]

So is it allowed or not?

If not how about a Boss Mustang?

I'm looking for a shell that I already have to fit a Slot-it chassis

The Merc C Klasse replaced the 190E in manufacturing terms in 1993. But I don’t know of any National Rally History. So you need to do a search and come up with some evidence ?
The Mustang Boss is in the same camp, I have not seen it listed on the website I used, so you need to find some evidence.