Welcome to the forum
It's pretty quiet around here nowadays but there are a few offs still kicking about on here. There is a bigger following on the Facebook page so worth a look.
Best bet would be finding someone who is stripping an Si/xr2i for parts. there are a number on the Fiesta-si.com Facebook page but theres also a Mk3/3.5 Ford Fiesta page which I follow and theres normally someone breaking fiesta on there quite often.
Certainly sounds like you've got a broken spring. If the top mount looks shagged then probe due to rust that both have gone together so your plan sounds like the best one.