Not sure if the spit from the old kit will work with a current Genesis. It has to be round in two spots in order to spin in the Genesis rotisserie grooves. Make sure it can spin first before looking for mounting hardware
The only way to get a correctly machined spit is from the Weber 7519 rotisserie kit, or the specific weber made spit from OneGrill.
If you have a spit that isn't rounded in the correct place, that problem can be fixed with a metal bushing. I believe they have them at OneGrill.