If the Automotive arm keeps growing at the rate it has been over the past few years then I think a McLaren-built engine becomes a more realistic proposition (depending on the 2021 rules though I guess).
They have knowledge of how Honda & Renault (two of the worlds largest manufacturing powerhouses) have built their units and already have experience in the turbo-hybrid field with the engine they built for the P1 and the Senna.
If it were to happen it would be a huge risk both financially and reputationally so I’d imagine they’d maybe do it alongside or, at the very least, bring in a few top guys from Cosworth or Ilmor or something to lead the project.