YEp what he said, unless its a show bike and its a pre 75 and you want the look, going racing you will need the 21 - 18, but drop the forks down until they hit the underside of your bars.
Here are the usual 4 - 19" choices for you - these girls are on 19" , my trackmaster ( purple one ) on speedways, white one has carlsie rear and goodyear front and my 51 bike is on the Pirelli MT53. The carlise and pirelli have ended up the best of them, for some reason its like an early trails intermediate pattern but seems to stick but also cuts through dirt soft track ( afternoon heats) the speed way sucks unless you have the HP to drive it and the goodyears just look cool, but alas when the tracks chopped up you just bog down or they float off course.
Like firko said its a given, 18 with a Michiein if you serious, plus proberly cheaper in the long run if you doing a budget run and you get more satisfaction.
next year im doing a rejigg and the race bike will be on other rims, and ill re shod these 19" with there orignal fat tyres for show - display events.