how to replace tires to a long out of production kit

I have a very old 1/8 scale kit that was discontinued back in the early 80's.Actually, the manufacturer of the kit went out of business/bankrupt, or whatever you want to call it and the kit has never seen a re-release since then and probably won't ever.The kit is a 1/8 scale Entex kit,the 935 RSR Porsche (racing Porsche 1972-1974).The kit is a bit rough,but considering the $$$ it costs to buy this kit in really good condition,I couldn't pass it up.I am missing all 4 tires to it,and I don't know where to find 1/8 scale rubber tires that will fit the rims that came with the kit.Does anyone have any ideas?