I used to have an R2137 (the one with the R12 engine and upside-down gearbox). Funnily enough, the R2137 takes an R4 alternator, not an R12 item.
There are some tyres on ebay at the moment that should make a good conversion for an Estafette: http://cgi.ebay.fr/Lot-de-5-Pneus-o...yZ109285QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Anything I can help you with (check the fuel jet!) just post.