There's a product you can get from car accessory shops called "instant metal". It's a resin with aluminium filler that sets and can be machined, drilled and tapped etc. just like a light metal. I think this would be ideal for you job.
http://www.cotronics....o/cotr/rm_putties.htm
Second one down.
Graham's suggestion does look good, but it'll be rather expensive I guess, if the void in the hollow heads is very big. Assuming these are cast plaster heads, then all you really need is to fill them with Polyfilla, or even sand & cement.
Either will take drilling and Rawlplugging.