I can try to answer a few of these for you
How do you get rid of pony cancer?You really can't get rid of pony cancer, it's either a mold or plastic breakdown in the pony's body. Sadly, a few ponies are especially prone to it, like Gingerbread and Cherry Treats. All you really can do is leave it alone, or some people tend to paint over the spots to make it less obvious. There's also some debate about whether or not it can spread, so some are cautious about storing cancerous ponies near minty ones.
Can you give Twinkle Eyes their finish back? No one's found a true match for the twinkle eye finish on the originals, but many have used clear or very light pearly nail polish for a substitute finish. It's not exactly the same, but it makes them look less like zombies
It's also recommended to test this on baity TE first, just in case it comes out wrong so you don't mess up their eyes. If you do mess up the finish, you can't take it off with acetone as that could melt their eyes.
How do you get rid of more permanent pen/Sharpie marks?For deeper pen marks, try sunfading! Over a period of time the plastic in the pony's body reacts to the sun and "bleaches" it, getting rid of most pen marks. You have to cover the pony's symbols and eyes though, as they'll fade in the sun too. There are several tutorials all over the Arena, varying from covering the pony's painted parts with tape, wrapping the pony in plastic wrap and"drawing" over the paint in sharpie, or using cheap acrylic paint and then removing it with rubbing alcohol. If you search, you can find a of tutorials easily enough. There are certain pony body colors that can react badly to the sun. Again, Gingerbread and Cherry Treats tend to "tan" and turn brown, and blue/purple bodies fade weird. Also search for a list of ponies that don't react to sunfading well.
For any other pony refurbish questions, I suggest checking out the Pony Preservation Project! It has tips anywhere from how to get rid of bad smells, custom dying tips, and more detail on all of the above topics.
http://www.mlppreservationproject.com/What type of clay do you use to sculpt on ponies?I am not a customizer, so I don't have absolute experience on this, but I've heard that many people use Apoxie Sculpt and it works really well, is easy to get, and is relatively cheap.
A really good custom tutorial site I've seen is Aikarin's. She's a retired customizer, but her website is still up.
http://www.aikarin.com/mlp/customs/customtutorials.htmlMore information on sculpting in general, and a compare/contrast chart between different clays: (
http://www.aikarin.com/mlp/customs/customsculptingfaq.html)
How to turn a normal pony into a merpony: (
http://www.aikarin.com/mlp/customs/merpony/merponytutorial.html)
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