Normally a well fitted contra-piston doesn't need a O-ring to help seal it. You can see that with many of the diesel engines that have been made over the years.  But if you use a Viton O-ring it wouldn't really hurt anything either.
The 1/2a engines were using O-rings on some of the different brand heads, mainly to help cut down on the time it takes to manufacture the heads in order to sell them for a better price.