Have: old 15" Marshall cab. Need: new speaker...but what?

[stevebasshead]

I have an old unloaded 70s/80s Marshall 1x15 bass cab. Originally it had an 8ohm 100w Celestion Powercell but this is in need of repair and I suspect a recone will be about as expensive as buying a new speaker once postage costs there and back are taken into account. But I'm not sure what speaker will work well in the Marshall enclosure and I'd like some advice please. Is there software that can reverse engineer what speaker will go with what cabinet? I've only seen it the other way round unless I'm mistaken?

Cab [u]internal[/u] dimensions are 28" high x 28.5" H wide x 11.5" deep, that's about 5.3 cubic feet/0.15 cubic metres. It's probably not the ideal shape for a bass cab but for half a dozen reasons I'm keen to make use of it.

The speaker mounts to the baffle which has a wide oval port/slot at the top. If needs be I don't mind getting a new baffle made up with appropriate porting to suit whichever new speaker goes in.

I'll be using it with a 100w Superbass head. It will be used as on-stage monitoring for me as I'm mic'd up though the PA for FOH but I do like to drive the Marshall a bit to get some valve growl going on.

Any ideas what I should be looking for?

Thanks,
Steve.