Hi Andy,
welcome at Stridaforum 
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According to drawings had these elder models a recessed snubber bolt.
(Function in short: Open bolt - put feeler gauge between belt and snubber - tighten bolt - remove gauge.) -
The freewheel inside perhaps, but not the plastic part.
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No, cant’ be replaced (riveted) and not any longer available (for metal wheels, plastic ones can be found…maybe…)
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Possibly…pics could tell more.
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As long as they do not touch anything (belt, frame) they’re not too wide I believe…anyway I’d recommend to use the same tyre sizes/types front and rear
Of course you can send pics to be uploaded also to the address below,
Cheers,
Chris