There used to be a variety of kits to convert Mauser, Springfield, Remington and Winchester rifles to left hand. Some were mostly add on parts, others required welding and machining. All required machining on the bolt.
Those pretty much all disappeared in the early 80's when manufacturers stopped their campaign of hate against the left.....
I have done two left hand Remington 40X conversions for benchrest shooters. It is pretty involved. Get or make a left hand cocking piece, cut the bolt in half, fabricate and put a new back end on with left hand handle, make filler blocks for the rear bridge and weld in, machine the rear bridge to left hand geometry for the bolt clearance and extraction cam. Decide if it gets a left hand safety. I suggested Gentry 3-pos safeties, but neither shooter wanted a safety. There is a little fitting of the bolt head so it engages the receiver in the opposite direction.
They turned out really nice. The why was that they had everything dialed in except the action and it would have cost 4X more to build new than to convert. A little stock work to fill a bolt handle cut and make a left hand cut.
If you are starting from scratch......If I were to quote the work, it would be more than a quality left hand action.
No idea what it takes on any other action. These are the only two I've every even considered.
Jeremy