Could prove difficult to get an "off the shelf" spring and guide kit for the Mercury, but Knibbs makes a rear guide that is supposed to fit better than the original, just make sure to find a spring that matches.
Welsh Willy kits are made to order and will take a while to get to the customer but they are well made and are basically drop in with no spring shortening needed.
Pete
Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in