Marty, The CSB250 box converts sine/cosine to nmea. If you can't find one, you can switch to the latest compass, RFC37 fluxgate and do it that way. It has both sine/cosine & nmea output, The RFC37 is worth considering as it is far more accurate and responsive than the old RFC250 anyway. (the RFC250 is neither fluxgate or gyro, it is essentially an electromechanical fluid filled magnetic compass with internal sensors. I changed mine last year and the difference is essentially a new autopilot. Cost was about $320.00 new off ebay however some are trying to sell for double this.
Simply clip the wire at the 250, use the old wire & connect to the RFC37 sine/cosine terminal strip and your nmea to the nmea terminal strip. Done