I am completing my final year undergraduate study in Mechanical Engineering. I've had to have a scope change because my last project was not going to work with the pandemic. I've been given a project in molecular dynamics by my supervisor to be completed in 2 months (slightly reduced scope). I wanted an outside opinion on whether molecular dynamics falls under mechanical engineering.
In my opinion, it is very difficult anymore to get to anything truly novel without mixing different disciplines. For example, its difficult to imagine, advances in molecular treatments, without the interaction of biologists, statisticians, engineers (electrical, mechanical and chemical).
So in my opinion compartmentalization of disciplines is something that limits the advance of knowledge. So in that respect, I believe that you should be open to different experiences and fields of study.
Suggestion: Like SolarMike suggested, you are in a tight spot. If you are remotely interested in the subject, I'd suggest take the bullet and complete it to the best of your abilities. Final year projects are rarely important to the professional career. I think your supervisor, is probably trying to get you out of the tight spot, so you should take up the offer.