The original drivers were manufactured by Audax (FR) / Polydax (US). Audax no longer manufactures replacement woofers. If you could find Spica graded new old stock (NOS) with the proper service codes and model number that would be ideal, perhaps just as good would be to cannibalize a used set with the same service codes. Other options include non-graded, NOS Audax (FR) / Polydax (US) replacements, which turn up on eBay from time to time, or getting your driver rewound by a rebuilder. If you go with non-graded drivers or chose to have your drivers rebuilt, there a strong case to have your drivers redone in pairs.
Note: If you are getting TC-50s off ebay for parts, be aware that the woofers, tweeters, and crossovers were not the same between early (TC50) and late (TC50i) models (see revision).
Tweeters were not graded on Spica's so these can be changed for same model number driver without issue. For the TC-50, Madisound offers the Audax 4 ohm TW025A2 driver (specs) as a replacement tweeter for early and late units. Visually (based on the length of the voice coil), these appear to more closely match for the earlier the HD 100 D25 tweeter. You should note that the terminals on the TW025A2 tweeters are located 40 degrees apart (if the speaker were the face of a clock the terminals would be at 5 and 7 o'clock). The terminals on the original TC50 tweeters were 180 degrees apart (using the clock again, the terminals would be at 3 and 9 o'clock), so some cabinet modification will be necessary. As these are not an exact replacement I recommend you replace them in pairs rather than mix this driver with earlier models.
Rebuilding \ Repairing Drivers
The voice coil of woofers and tweeters can be rewound by a competent speaker rebuilder. However, if you have a physically damaged cone or rubber surround you may be out of luck. Everyone I have contacted says the butyl surrounds for the woofers are no where to be found. This means a rebuilder will use foam surrounds. Foam surrounds will significantly alter the mass of the woofer and its sound.
Speaker Cone Doping
The woofers of the TC-50 and 50i were doped in a mix of Elmer's glue and water. According to the (SLUG Post 836), the Elmer's glue is mixed 60:40 glue to water (3:2). They first placed an 1/8" bead is put around the base of the cone and let it dry, then cone is coated. It is believed that the TC-50s had one coat, while TC-50i's had two coats (SLUG post 160).