Before you launch in to repairing your Spica's, there are a few things you need to know. Spicas were special for several reasons.
Spica designer John Bau took great care in the design of the crossover. Where most speaker designers are focused on maintaining a flat frequency response, Bau went on to ensure time alignment. Time alignment means that regardless of the frequency produced, the sound emerge from the drivers at the same time. This is very difficult to accomplish with any crossover design, but is critical to producing the precise imaging for which Spicas are known.
In traditional speaker production the crossover would be modeled on a computer and then several prototypes would be built to work out the real world differences. Then production models would then fall with in a range, based on variances found in component values in production. One of the higher-end speaker builders will go the next step and use close tolerance 1% components to hold production models to close tolerances.
In Spica's case, standard tolerance components were used, but were hand matched to ensure uniformity. This held the Spica's crossovers to extremely close tolerances. However, Spica went even further, matching woofers with a TDS spectrum analyzer for overall sensitivity, frequency, and magnitude of response. Woofers were then assigned a service code, paired, and mated to a corresponding crossover. This grading system assured that the left and right channels are perfectly matched across the frequency range. However, this grading system also means when drivers are replaced, matching driver models and service codes should be used.
What if the service codes for the woofers don't match?
I don't want to be too crazy here, if your choice is working Spicas versus the dumpster, you should be open to some compromises. Early on I had a problem with my pre-amp. It would occasionally burp DC and fry a woofer. The replacements from Spica were around $80, plus the time and cost to ship. I found a local source for a generic Polydax woofer ($25) and replaced one side with the same model, but a generic non-graded version. While the precise imaging would not have been maintained with this compromise, the speakers were still time-aligned and had similar volume over much of the audio range. Not as good as new, but even without the graded driver they still sounded great.
What about the tweeters?
All indications are that the tweeters were not matched. At least one source reported the tweeters were so close that the additional matching step was not required. In support of this, tweeters did not carry any service codes, nor were service codes required when calling the factory for replacements.