my mugo pine is gorgeous but now is to big for its location – can it be cut down to size or is it scrap & start again.
none really - question says it all.
A mugo pine won't take well to major crown reduction pruning, you will bare the stems which may not resprout. You may be left with ugly stubs and dieback, and maybe the beginning of the end.
Light crown reduction is possible by an experienced professional. Or consider transplanting to a better location on your property, or selling/trading to exchange for a new, smaller plant.