In the UK there are basically two groups of owners, those that do concourse restorations and those that modify then to make them more practical. (although many have one or two of each)
Mine is fitted with a tuned 125cc and tops out close to 70mph, which feels quick on 8" wheels.
Engines that just bolt in now available up to 300cc, but there is one with a sohc 750 in it out there somewhere.
On the right roads they're great fun to ride.