If you port a set of p5 or p6 heads as far as they will go and fit the biggest valves they will take they still won't flow as well as a bog stock set of SD1 or later heads so no don't bother.
Mike
I shouldn't bother mate, there are hundreds of them around and they can usually be picked up for a tenner or less! Clean 'em up and use 'em for porting practice imo.