Comment Run! (Score 1) 424
Fast. Do not look back, lest you turn into a pillar of salt!
Seriously, if you're over 30, quit now. Otherwise, if you are young and inexperienced, stay if this is a WELL-PAYING opportunity or you REALLY enjoy and trust yourself or the company.
If you stay, start MIGRATING the pieces into something you understand and can document. I recommend migrating to a mainstream, well-supported, open source projects UNLESS the proprietary alternative is vastly easier to deploy (this is generally not the case). Test each step and have a backout plan ready. Backup whatever you can.
Realize that you might succeed and prove yourself to the company. Also realize that it might all come crashing down on you, with all the blame being assigned to you for any and all things that go wrong. The pitfalls are many. Use the source. Good luck!