filling in cast heads

The liquid ferrous or non ferrous metal that you might be trying to fill in the cavities with would probably freeze or set up before it flowed to every location you need it to. If i am reading you right. To do it solid the head would have to be heated in an oven to a very high temp then it might still crack.

A specialize high perf machine shop could accomadate you but at a price without filling the head. There may be other options if you were willing to let the cat out of the bag and explain exactly what you are trying to accomplish.

Hope this is of some help.

If you have a single port feeding two intake valves you need a good turn on the back wall of the port. What is happening with the head, (not aquainted with the D21) the velocity slows when it reaches the two bowls that have also been opened most likely and the fuel falls out of a state of suspension. Unless you are really turning the RPMs up you might be supprised to see how it ran with quality flow instead of higher flow numbers. Where the port ends on a lot of heads at the two bowls there is usually a sharp edge that makes a 90 degree turn left and right to each bowl. What needs to happen here is a large slow radius and make up the differance in the port on the back wall with a divider/director that maintains the port cross section and also helps turn the flow down to the chamber and will not allow the flow to slow. This higher velocity will also help fill the cylinder across BDC.

After this is fixed you will need to do something about the exhaust ports.

This is just an opinion, take it with a grain of salt.