Your role in the project is important. If you are a technical resource, it is not your job to "rescue the project". If you are the project manager, it is different. If they want you to do both...run?
It sounds like you are on the technical side, since you talk about the technology stack. If so, I would look at this as almost a win-only case. If your skills are sound, and you are motivated, you should be able to do something with the technologies. To your advantage, the bar seems pretty low - failing slower is an improvement!
As long as you have been clear about you strengths with the technology, you have set the expectation that there is some learning involved. At a minimum you should expect some time to improve your skills on your own, but you could also negotiate some training. Tell them you would like them to bring in an expert for one day to give you some 1-on-1.
Accepting the position shows you are a team player. Discussing your concerns in the process and proposing risk mitigation shows you are a smart team player. If the project succeeds, you are part of a turn-around team and people will remember. If you are successful and "save" the project, the rewards are even greater. If the project continues to fail, and you are not adding to the problems, it will have a minimal negative effect on you.