Personally, i can see McLaren producing its own engines in the future. I can see this being done in about 3 or 4 ways.
Posibility 1 = Buy Cosworth for a sum of money, and split the company into 3. F1 engines would be one area, Electronics would be consumed by McLaren Electronic Systems (MES) and all Cosworth electronics would become part of MES, thus becoming like Magnetti Marenelli, and the third devision would be still for supplying other areas that need engines like Motorboats.
Posibility 2 = Buy over a company like Zytek, split it in a simmilar way to Cosworth, but also use them to produce a F1 engine. But heres the curveball, use the Zytek arm to compete in Le Mans and compete with the posibility of Ferarri and McLaren with LMP1 or LMP2 cars in there alongside Lola-Aston Martin, Peugoet and Audi.
Posibility 3 = Buy over IP for a engine manufacturer that has left the sport, either BMW or Toyota aint gonna be hard to deal with, a sum that is posibly gonna be in the teens of millions, but the thing with that is start up costs for a engine plant. Not to mention staffing needs. This method would also purchace all powerplatns that manufacturer also has working at present.
Posibility 4 = Start developing their own cars (whitch they have started) and then once thye payback has started happening, then start to develop the road based infrustructure to support the F1 engine side later, this is the most long term of the views, but could be the most probable.
Personally i think its only a matter of time for us seeing a "Scuderia McLaren" car on the F1 cuircit, but the thing is when. In my Opinion i think it could be as early as 2016, but it is all down to Ron Dennis and what his vision is. Its only a matter of time, it will happen, but is down to the money situation, and what Ron wants, thats the way i see it.