You should be able to pick up the old M81190 Seatalk fluxgate on EBay etc for around a hundred quid or so . That will solve the problem entirely if correctly sited
Raymarine do still make and sell compass transducers but not for Seatalk 1 and therefore not directly compatible with the ST series tiller pilots
(and to pick up a minor technicality, chart plotters, GPS etc cannot supply heading information)
