If I were you, I'd rewind it, if you have the equipment.
If you're just looking to replace them, would the schaller bass humbuckers fit? They have extra screw lugs and seem to be the right size(68x37mm). If the two screws on the one side are 8mm apart, it should mount just fine.
http://schaller-guitarparts.de/hp135071/Bassbucker.htm
Edit: Rio grande also has some that look to be the same dimensions as the schaller(including the extra screw holes)
http://www.riograndepickups.com/scart/Prod...ctName=For+Bass
Edit 2: There's also this from SD, but it'd require a modification to the mounting ring like you said http://www.seymourduncan.com/products/cust...awkbuckervario/
Also any guitar pickup with blades should work just fine(though probably not as well since they're not optimized for bass). Or if you really want to get crazy, make a new mounting ring for a strat rail pickup. The lipstick tube humbuckers from GFS might do well, too.
Haha, sorry if I've overanswered your question. I was intrigued and searched around.