That is exactly the sort of thing; a tiny length of brass rod should do the trick.
Be careful putting the back block retaining bolt back in, it is VERY easy to misalign it slightly and cross-thread it - a hard steel bolt going into a softish alloy block wasn't one of FWB's best ideas...
According to popular myth, the mainspring can be replaced with one from a 77 if you have one kicking about.