will we be able to purchase key compoments such as engine plans from another manufacturer once the full game is running?
(like rover did with its aluminium V8)
to save development cost, and to acquire high tech powerplants as a low volume maker…
Yes! That is a planned feature, although it probably will be added very late in development. There is supposed to be an engine market which you can buy from and sell to. 
wonderful… I like engines with fascinating histories like the Emi-sul V8 by simca
Will tuning of these crate engines be allowed? If not full structural modifications like the block and head tabs at least the fuel and exhaust tabs?
I would say so, but there would be extra expense and time in making changes compared to changing your own design
also cams forced induction ingintion and all the “small” things