How to publish my PhD as a monograph when examiners are unable to oversee the process?
Be upfront and honest, and ask!

Make contact with these select few. Explain that you've received interest in producing a monograph, but those doing so later declined their support because their qualifications or subject knowledge are, described much lesd tactfully than you should, inadequate.

Describe your reasons for contacting them in particular - yes, personalise these requests. Ask if they are available to lend support, and whether they would desire to do so. Don't send a copy of your thesis, but make it clear that you will on request, that is, if they don't indicate that they looked it up already.

If you get declined, thank them for their answer, and for any helpful critique or advice they give. If they accept, discuss with your university to see what support you can get from them -  if you have their backing already, this is a good thing to mention in your approach to potential collaborators.