great find! Brownburst and rosewood is one of the less common varieties. Pickups pop up from time to time on on ebay.
You probably want to find a t-40 pup, but if you are looking for alternatives Dark Star pickups sound just amazing but are pretty expensive. Also, Dimarzio makes a "Will Power" neck position humbucker. It's similar to their model 1 gibson humbucker replacemnt, but has a slightly wider string spacing. With either of these pickups, you may want to experiment to see whether to put the replacement in the neck or bridge position.
Best of luck.