The crossover shown in your picture is very simplistic and offers very little protection for the Horn Driver. I don't believe any of the speaker components were manufactured by Sunn, although they may have been a custom order from a speaker manufacturer.
According to my info, the Sunn part number for the Horn driver is 81-0294. I can't really read all of what it says on the back of the driver in your picture. Try looking online to see of the company is still in business or who may have purchased the company. Sunn says the replacement "cartridge" was part number 81-0285. You need to know how the driver mounts to the horn. The driver may screw on to the horn ( 1-3/8 - 18 TPI like an old PA driver), that would make finding a replacement somewhat easier. Parts Express has a selection of drivers that might work. There may be a similar driver used in one of those 4x10 + Horn bass cabinets.
The 1979 catalog gives the Sunn part number for the woofers as 81-0137. The 1982 catalog gives the part number as 81-0246. All it says in terms of specs is that each speaker is 16 Ohms. A typical 12 inch guitar speaker will have a peak in it's frequency response usually in the 2KHz range. This makes them less desirable for service in a PA cabinet. A speaker intended for PA service will have somewhat lower efficiency, but a more controlled peak.