Seymour Duncan, Gibson, and others (I am sure) make direct replacements for the P-90. From the Seymour Duncan list, I would probably suggest the SP90-3, as it seems most likely to have the sound you want.
I would recommend you avoid trying to put in a standard sized humbucker, as the pickup cavity in your guitar is both too narrow (and therefore needs routing) and too long (which means we would need to fill in the excess length). It would be quite expensive and quite difficult to do that job in a way which would look good.