Hi Phil,
Inrad sells a 6th-order 47.055MHz crystal roofing filter meant for the FT-980, but I don't know anyone who has installed it (and certainly no one who has done pre/post testing to see effects on sensitivity). Cost is $125 (plus S&H), and it is currently in-stock.
I will include some details in the "mod" files folder. Give me a couple hours ...
Note: You will almost certainly limit FM bandwidth to 5kHz (vs the 12kHz native bandwidth). While the initial tolerance of the matched crystals will likely be good, I'm not so sure how a VHF filter is going to behave over time as the crystals age. At 47MHz holding a 5kHz band requires VERY closely matched crystals.
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-Mat Breton, N8TW
Mat Breton N8TW