Hi again! Yes, I am suddenly busy again with my machines after a long absence

I'm looking for a mainspring for my 1937 Remington Noiseless Portable (serial N121425).

I broke it myself whilst trying to fix the broken drawband — basically the most heartbreaking moment in my life thus far! 

Also, is it ever possible to fit a mainspring from a different model? If it's the same size and shape?

Thanks in advance for any help.