huh... strange.. very strange...
well, I have no idea, but I've seen some scorpion (we talk about spider carts, right?)
with non-kart wheels on the internet.. some with rather large wheels;
so, maybe a golf cart wheel, maybe a lawn tractor, maybe even a car wheel... I have no idea.
12" is a lot! well, at least for a wheel diameter less than 20"...
(metric that's a 305/xx - yy ; yy being the rim diameter in inches xx the height in percentage "50" for example would have a 6" sidewall height)
They did it?, ask them...
Sorry.. but as long as we don't know what wheeldiameter (or rim diameter) we're talking about it's pointless to make a suggestion.
But one thing I can tell you, with less than 10" wide rims you'll run into troubles, you'll also will with rims wider than 13" maybe 14"
So, dial in your rim width as close to the tread width as possible! (11.5" - 12.5" ideally)
'sid