Changing the jet is easy, just take off the float bowl from underneath the carb, you will see the jet there...a brass hexagonal thing with a hole in it and a number on it, i would say only buy genuine mikuni jets. just see what size is fitted and buy a couple larger ones....like I said earlier i'm thinking 250 will be about right depending on whats already in there.
Allens sell them cheap .........google allens mikuni
Take care undoing the float bowl screws, they are easily chewed if your driver is not a good fit!