Yes, you can fit a Monty tire in a 20" Pashley frame.
Personally, I think the Pashley trials unicycle is the best looking one,
although I don't think its worth the extra money if your buying it new.
Since you already have the frame, though, I say go for it.
You will have to get some special tools to remove the bearings from the
lollipops, and possibly to reinstall the new ones. You may have to get a
bike store to do it, or just send the frame to unicycle.com and get them
to install the wheelset.
Hope this helps
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