In order to fit a Palitoy head the helmet would end up too thin, also the plastic used for Palitoy heads is very rubbery and not easy to work with ( especially when it becomes thin)
If you do try be careful not to dremmel for too long, the helmet will melt !! ( it doesnt take much heat to totally ruin it)
I have tried before with no success, hopefully someone else can prove me wrong ?
I think Paul did an SAS one, but it was for display purposes only, It looked great in the figures hand but I dont think he managed to make it fit the head.