Aye Jez,
I have a BMW F650 head bare at present and could do some measures for you.
Whilst the Pegaso has the 5 valves, I understand the BMW was purposely kept with 4 and includes an extra oil feed to the valves instead of the 5th valve. However, this maybe just balderdash!
If you need crankcase measures might be able to oblige there as well as there is a spare motor in a crate as well!
Cheers, Bill