Don't own a pair of the speakers, but have you tried McIntosh parts for the tweeters? They have a surprisingly large inventory of parts to support older gear. If they do not have the exact tweeter, you might ask if they have a recommendation on a replacement tweeter. That failing, ask the couple of speaker repair vendors who adverstise here on AK what they might recommend. They may have a cross-reference. Will need to match the impedance and sensitivity to get proper balance out of the speaker, unless it has an L-pad in the circuit.
Accuphase T-106 Cary SLI-80 Sig F1, PH-302 Marantz SA8004 McIntosh MC50(x2), MR65A Mitsubishi LT-22 w/Lyra Kleos, Ortofon K-b & AT33R, Ortofon 2M Black & AT150Ti Sony TA-F707ES, ST-J75 Thorens TD-145 Yamaha T-85, TX-1000