You don't need to buy a whole new horn driver... there are replacement elements available for the 22A. In fact, the 22XT element is a BOLT ON replacement element FOR any of the 22-series drivers, 22A included.
The BW1505 is certainly a good replacement for the original Black Widow 15... but again, Peavey has replacement basket/cone assemblies for these. Cheaper than buying a whole new woofer! The replacement for these, if they ARE Black Widows in the cabinets, would be the basket for the BW1501, IIRC.
HOWEVER- that said- I've seen LOTS of SP1s- earlier versions- that had square-magnet STAMPED STEEL FRAME CTS and/or Eminence built 15" woofers in them! In the early days of SP1 production, there WERE NO Black Widow woofers yet- Hartley Peavey didn't design the Black Widow until several years into the production of the SP1s!! If it's these woofers, they can be reconed pretty cheap... I think we charge like $55 each, to recone those. No problem.
BTW: I have a crossover diagram for these, if necessary. They're sort of strange topology, but fairly easy to fix, if you know what you're looking at. Most common crossover problems (which, BTW, can result in woofers not functioning) are a shorted big electrolytic cap (frequently 33uf) across the woofer, or a dead 50w, usually 10 ohm or so, resistor (great big cement block resistor, on the back of the crossover). You might want to check out the crossovers, while you're in there...
With regards to the crossover point- I would NOT go to modifying the crossover. The horn NATURALLY doesn't want to go above 400 Hz, but the way the Peavey crossover is wired, it FORCES a bit more output between 400 and 800 Hz. It's really not bad of a response curve, if you measure it in front of the speaker. The effort required to re-engineer the crossover, is MUCH MORE than it'd probably be worth, in terms of any significant sonic improvement...
Regards,
Gordon.