A 'literal' translation could be:
si quando leges physicae valere desiverint, dominus te adiuvet.
'If ever the laws of physics will have ceased to apply (to be valid), may God help you.'
However, as ...
As far as I know, the names, especially the abbreviated ones you list, are always in English whether or not the surrounding text is in English.
Technical abbreviations like this tend to be universal, ...
A couple of suggestions:
Use fungi instead of agere. (This you did in your revised attempt.)
Use an adjective like in English. Imperiale is better than imperii.
I think the second part works better ...