I'm pretty sure the weber casters come with that insert. Buying the casters should be similar price to having someone ship you a used set. Amazon has them for $6: www.amazon.com/Weber-6414-Caster-Includes-Insert/dp/B000WEKMRU
The performers already have an 8" wheel so you will need a 10" wheel to raise it one inch. I've mixed up the radius/diameter measurement before and needed to find replacement legs for my frame after cutting the legs too short by 1".
Just a suggestion on a band aid fix on short legs
I havent measured wheel inserts..but just conceptualizing.
If the inserts are longer 2 inches..or you can buy 1 1/4 inch tubing...cut the one inch size and then ise inserts to provide internal support...probablynwould not provide a lot of sheer strength...probably wweak solution.
2nd option..cut leg shorter...and buy tubing...cut the replacement tubing to ample length so you can insert star nuts in both the short legs...and the cut to fit replacement tubing..then fasten together with 3inch 1/4 20 hex head bolt..you can drop a few star nuts into each segment to increase sheer strength....you would want to position the star nut to provide enough clearance for the wheel insert.
Again..not tremendous sheer strength but better than option 1. And probably good enough for standard handling.
Installing 2 star nuts into each section of tubing close tobeach other for total of 4 star nuts will increase sheer strength.
You can bondo the joint and then sand to flush fill...
This is a lot easier to sketch than describe via writing