I bet that the are more Morris 1000's around than BMW's of the same vintage, as for rust, those problems started when the government interfered and BMC was the result.
Most of the Morris Minors still around have had extensive rebuilding. My brother owns one and the standard tool to take with you when contemplating a purchase is a coal hammer, so that you can do a bit of pre-sales 'testing' on the frame. The comparison was of a Morris Minor and a modern BMW, not one of the same vintage. I have owned in my time various VW Golfs, the Mk1 were rust buckets, whereas I have had 4 Mk4s with not a spot of rust on 15-18 year old cars. WIsh I still had the Mk1 which I dumped as junk, it would be worth a bit now.
Back to the Class 21. The Worsley etches are nice, perhaps they could be paired with modifed Dapol Class 22 cabs to produce a model.