I have some engines like the Enya 19, FOX 35, Enya 35  that has the older wedge combustion chamber shape and my question is

if I get a custom made Hemi Head for them, what improvement could I expect?

Martin

Martin ,I tried hemi heads on many motors. I was a tool maker many years ago so it was easy for me to make them. I was trying for a special motor run not more power.They worked very well on the ST/40-46 and 60's.Some other motors they did nothing but make things worse. One thing that worked very well for me was to cut down the depth of the squish band and leave the head shape alone. This looks very odd but worked very well on the ST/46. In the old two port cross flow motor the  stock head is more efficient at using all the fuel and making power but that is not always the best stunt run. As Bigiron says it is a hard question to answer.
Ed

  Martin,  you are correct in the fact that I designed the Fox brand Hemi-Head for the Fox stunt 35.  There are currently four Fox Stunt 35  Hemi-Heads on the market, three of which have virtually identical head shapes and squish band configuration.  Only one is a "two piece" and that is the Fox mfg version.
 The opld fox was notoriously anemic when it came to "two Cycle" needle setting, but was very healthy in the four cycle needle setting, and also transitioned to and from the two to four settings 4-2-4 break setting.  Virtually all of the four hemi heads on the market for the stunt 35  enhance the four setting while controling  the tendency to run away when going to the "two" setting.  In my opinion,  the power increase in considerable when the right prop/fuel combonation is used.
  Noiw---  The Anya 19 is an engine that already likes to run in the "soft two" needle setting and does not like to 4-2-4 break as does the fox 35.  So I doubt that much (if any) benefit would be realized by converting to a Hemi set up.
  Depending on the shape of the baffle on the top of the piston, I think that it could be fairly easy to make a two piece head for them though.  If it slopes up from the exhaust side it will be very hard, but if it is square as is the Fox Stunt 35,  then quite easy.
  I do not do that type of work any more so I am NOT volunteering.

  Marvin Denny

Martin, you should check the labor rates for custom machining.  I think you will find that it would be far cheaper to buy almost any new engine than to have even one head machined. 

Good luck to you.  You have the right attitude and the right engines.  I have one of those old 19's and it is great.