We all have built that 10 year old OOP kit that we find is missing a part or has a broken part. One option is to try and buy a replacement, if you can find one. The other option is to cast a new one. I recently started one such kit and this is my solution.
Here is my problem. When I started to sand the tires, I discovered an air pocket. I figured I would fill it will super glue, but it was larger then I thought. So I cast a new one.

Casting resin parts

This is a 1970's era racing tire. As you can see the damage to the tire will make it unusable.

I used Alumilite Mold Maker for the mold. I also used their casting kit.

I used a 2-piece mold. The first step is to make something to use as a mold box. I use a simple plastic container. If you make your own, make sure to seal the corners so the rubber doesn't leak out. Mount the master in the center of your box.
I use rice that I pour into the mold box to give me an idea of how much Mold Maker to use. You then measure the rice and mix the Mold Maker. Since I made a 2 part mold, I only had to measure the bottom half.

In this case.... the rice is 50cc.

Pour the Mold Maker into the mold box. Pour it into the corners and let it flow around the part. Use some kind of mold release on the bottom half, I coat it with a thin coat of Vaseline, and pour the second part. You will need to make alignment areas to help align the mild halves. I use wood dowels set in the rubber.
Once the rubber sets, pop the mold out of the box and this is what you should have. Cut out a pour hole on one edge. Clamp the two halves together and you're ready to pour. Don't squeeze it too tight or you will deform the cast.

A bit on mixing the resin. I use small cups like you see in the hospital. Mix the resin 1 to 1 and mix it to a uniform color. Work fast if you are using the fast setting type. You will only have a few minutes before it begins to set. Once mixed pour it into the mold. I sometimes use a pipette to put the resin in the mold.

The finished product:

Here is another example of casts that I made. These are 50's Roadster tires.

I use the quick setting resin, but there are other kinds that can take up to 24 hour to set. I think the slow setting resin gives better detail as it has time to work itself into all the cracks and crevices. There are also different colors... I have used the clear, white and tan. The clear can be tinted to provide color.

While pricey the Mold Maker is good for perhaps 7 or 8 molds depending on their size.