Ooh I can offer advice what not to do, having experienced this before...
I knew of the ketchup theory, so took the cat and a bottle of ketchup to the bathroom. Dd16 (and the cats chosen human) helped. Amongst our disgust at the smell and the giggling at finally being able to sing Phoebe from friend's smelly cat song for a valid reason, we battled to stop the cat licking her self clean.
The cat was fine with having the ketchup slathered on, and I thought we were onto a winner. Then I turned the tap on...the cat turned into a whirling dervish. As we tried to add water to message the ketchup in, Dd got scratched to ribbons.
The cat got free and ran all over the bathroom, a million times. Thankfully we had the foresight to shut the bathroom door before the attempted procedure.
Eventually we managed to catch her and gave it up as a bad game. We wrapped her up in an old towel and promptly deposited her outside, complete with ketchup still on her fur.
When I walked back into the (white) bathroom, it looked like someone had been murdered, there was red splashes everywhere. On the fittings, on the walls, window, mirror, even on the ceiling. Some of it was hard to distinguish between ketchup contaminated with fox poo and DD's blood.
It took an age to clean up, and maintained a pink hue in places until it was repainted.
The cat clearly got the hump and was not seen until the following morning when she appeared for breakfast with no sign or smell of fox poo, but pink stained fur where she should be white.
Dd still has some scars.
So the best I can offer is...good luck.