I'm not actually sure how to help you - from what it sounds like, you have a third-party payment gateway for WP eCommerce that the third-party has stopped supporting?
It may be worth digging into setting up a staging set and running the latest WordPress on it with your payment gateway - just to see if it does in fact work. Just because it is no longer supported does not necessarily mean that it wouldn't work. I can't stress this enough, though - do this on a staging site! Not on your production server.
If that does not seem to work, your alternative options would be to hire a developer to rebuild the payment gateway or to seek out an alternative payment solution. I'd be happy to give you a quote on development for that if that's the route you choose.
Hope that helps!