Not a huge problem.
You can buy any left handed 300 Win Mag and simply re-barrel it. In some cases you'll want to reinforce the stock with cross bolts too, but it's easy quick and the cost of a re-barrel is not very high. If the barrel you remove is in good shape you can sell it on line, and recoup some of the cost of the conversion.
I have done 5 that way, and it was not ever a big job to set the left handed customers up with a guns they were happy with.
