[C320-list] Knot meter transducer

[Crosby Roper]

The paddle wheel on my knot meter transducer has corroded badly since I last used it. I checked on the raymarine site for a replacement and could find nothing; West marine seems to sell the whole unit for between $200-400. I'm looking to just replace the insert part rather than the insert and sleeve. Can anyone point me in the right direction?
Crosby Roper VMDTethys, Hull # 1054