I don't know about a "neck gasket replacement kit".
I've used AquaSeal, with the new gasket over the old one (Cut down to about two inches) and fitted over either a lampshade, a bucket or even a large enough bottle.
If you can spare not to have your old suit for a week, then send it express to Jamie Dors at Paddle Sports Repairs (in a shoebox works best).
He's really good and his prices are very reasonable and he'll send it back to you Express in the same box you send it to him.
Here's his link:
http://www.paddlesportsrepairs.com/Good Luck,
Paul