The cheapest way would be to drill the threaded holes out to 5/16 then tap them to 3/8,buy 10 of 3/8 x 2 1/4 unc socket allen head cap screws, a bolt shop should be around a $1 a bolt the ebay ones are about a $1 each but postage makes them not worth getting.
Drill the cylinder head out to 3/8. That's what I would do.
