You might want to reconsider using “welching” in this context. And don’t change it to “gyping”, either.
I'm happy to hear your data point that "gyp" may already have fallen into disuse; or did you mean that you now intend to start using it now to signal your non-PC-ness? If so, others that I recall hearing in childhood but not lately, which you might also consider adopting, include: "indian-giver" (one who reneges), "jewing down" (bargaining excessively), "n....r work" (which seemed to mean "hanging around and not working" back in the day, but now Urban Dictionary has it as "easy, repetitive work").