I just buy the coupler with base when the grandkids mess them up - I include them in my Lionel order list, which I grow until it is enough for the flat shipping fee or I need something now.
Edit: added link to Series 1 parts - Series 2 coupler may be different - I get the series 1 for the early set of the Remote Thomas
There has been a 50% parts sale online that coincides with the Open House in November.
Even if you do cut and grind, Lionel doesn't sell a similar spring? Do you have source for that spring?