Often you can turn the old tires down on a lathe and get a good new
surface. One alternative is to make tires from an innertube. Cut a
band which is wider than your wheel, but smaller in diameter. Stretch
it around the wheel evenly, then insert dowel pins or other spacers
evenly around the wheel between the tire and the rim. leave spaces
between the dowels. coat both the rim and inner tire with contact
cement. After proper curing, remove the dowels one at a time from
oposite directions and stick the tire to the rim as you go. When the
cement is cured you can trim off the excess tire with a stanley knife.
(OR, you could buy some tires,). Either way, its best to true up the
old surface first, a lathe being the easiest way.
Paul