Yes there were wider and longer factory Target hammers made for your gun, although these are somewhat harder to find these says.
These were listed as "Target hammer, .500" wide".
These will often state "stainless" although as above, they're actually hard chrome plated.
If you can only find a hammer in standard color cased finish, most of the gun platers offer special pricing for small parts like a hammer and you can have it plated to match the factory look.
Gun Parts have the narrow "Service" hammers and the standard wider "Combat) hammers available for the Model 66-4. part number 46.
When changing the hammer, remove the double action strut from the old hammer and install it in the new one.