German outlet Auto Motor und Sport conducted an interview with Mario Illien himself. Relative late in part 2 of the interview he was asked about involvement with the 2026 engines and he replied that there were multiple inquiries but nothing advanced enough to talk in detail.

In a bit of fun speculation I'd say there are three somewhat likely candidates:

• Red Bull – they approached Ilmor to improve the Renault engine when they were Renault's customer
• Honda – they sold their UK operations to Red Bull and already worked with Honda on the last-gen IndyCar engine
• Cadillac/GM – The current Chevrolet Indy V6 has been co-developed by Ilmor.