Upgrade Dream 250 to 305 ?

[sky.fred]

I'd like to gain a little torque on my C72 and upgrade it to a 305.
I've found that the upper crankcase has to be redesigned to accommodate the 305 cylinders, which isn't technically complicated.
I think it's better to fit a 305 cylinder head as well.
Did you carry out this modification, using which parts, and is the result worth the work?
Thank you for your advice.