Yamaha Riva Carb?

Anyone know of a carb swap for these?
My son's '87 Yamaha Riva 200z (200cc 4-stroke scooter) has carb issues. According to Jack's Scooter Shop, they're prone to such things. They tried to make it very easy for non-gearheads to start and ride the thing, but that made the fuel system complicated and more finicky as it ages. I've tried using Jack's tips without success. New gaskets, jet, inlet, etc. Yamaha wants $600 for a carb. In other words, they don't want me to buy one.
The scooter was a freebie, but it would save him some money getting around town. Ideas?

You might look into a Keihin CVK in a similar size to what the Riva has. A lot of 4 stroke scooters have used them. Recently, the European version of the Vespa GT200 used the CVK30. There was some Japanese small displacement twin (maybe a 250?) from the 80s that used this carb too. I've seen them go on eBay for about $25-30 for a pair.