If you want a conversion, first step is the frame. C-List, swap meets, yard sales, etc. This would be the most difficult, possibly most expensive route, as you'd need to be every part. The plus side is the bike will be exactly how you want, and ideally unique.

Otherwise find a used bike and get a new wheelset with a track hub.

Keep in mind buying an off the peg SS/FG bike will be cheaper.