would rewind it if possible. i'm assuming you already did a cross reference for the coil, but if you didn't here's the bikes that used that same pulser..
1974 DT360A CDI MAGNETO DT360A
1975 DT250B C - D - I - MAGNETO 400B - C
1975 DT400B C - D - I - MAGNETO 400B - C
1975 MX400B C - D - I - MAGNETO
1976 DT250C C - D - I - MAGNETO 400B - C
1976 DT400C C - D - I - MAGNETO 400B - C
1976 IT400C C - D - I - MAGNETO
1976 YZ400C C - D - I - MAGNETO YZ400C
1976 YZ400C CDI MAGNETO YZ400C
they do come up on ebay frequently.. if you look at completed listings there's about 15 sold in the last month...
you also may want to check the specs on the later one (77+) yamaha part #1M2-85580-20-00 - it's possible it has the same exact specs and just has a new part number.......
or you can check with electrex and see if they have a new one...