An engine development project for F1 is going to cost about 250 million euros, Mclaren cars sales turnover from selling 4500 cars will be 810 million euros. I dont see Mclaren simply being able to afford such a project.
Engine development needs a tonne of data, data that cannot be bought or that can be carried over by staff.
Engine development needs innovation something engine giants such as BMW, Illmore, Renault, Honda have gained by having a R&D team that is decades old attributed by a few cases Gilles Simon who add the leadership to the team but not the core strength.
Engine building is a core strength of companies like FIAT, Ferrari, VW, Renault, not so much for Aston Martin, Lotus etc.
F1 engine is an even bigger animal to handle, hence there is noway Mclaren is going to be able to start and succeed with their own engine project for f1.
If they want their own engines they will need to buy a smaller engine company and build mclaren values into it. Still finding the project for F1 is almost impossible and unnecessary.
Sure there is a difference in specing and engine and tuning what you have compared to casting, maching, assembling testing etc, but those are skill that can be bought on the openmarket.
Wonder which open market has metal matrix composites, AlBeMet technology etc in the open. BMW used cylinder linings from road cars for F1 engines - wonder if that is available in the open market too.