I only now from one guy who replaced the carb fuelled engine on his MK1 for a fuel injected engine. He got the 6th gear and better fuel economy as a bonus!
But I can imagine that's a rather expensive modification.
Its (a lot) cheaper to modify the existing engine, get a carburettor job done with Dynojet, and replace the stock Honda airfilter for an BMC or K&N unit, and the stock sparkplugs for Iridium ones (Iridium plugs are standard on the fuelinjected MK2). I had two carb fuelled Varadero's before changing for an fuel injection version in 2004; having the carb job done by an dynojet specialist, my fuel consumption dropped with 15-18%! Thats almost a fifth.
Prices: about 100€ per carburettor (plus the airfilter and iridium spark plugs, I would modify these in one go)
You still won't have a 6nd gear, but will save some money at the end of the day