I would have thought a call to Nelly at Cornerspeed or Alan at AJ RAcing would put you right.
If the route is 916 heads drop me an email, I have a pair that have just been completely rebuilt with new guides, valves, rockers, 4,000 mile old cams and shimmed ready to fit.
I dropped a belt in Belguim but re-fitted SP heads and had these rebuilt as spares, but they have just sat around since.
Good luck and would be interested to see how she performs after the Big Bore kit.
I opted to put in DP high comp. pistons at 916cc in my Senna with SP heads and some nice cams, a few other bits and pieces like vernier gear to dial in the cams etc. I got 126 bhp at the back wheel, was considering 955 or 996 so would be interested to see how yours goes.
The work on mine was carried out by Alan at AJ RAcing, top bloke and has been looked after since by Nelly at Cornerspeed, so she has been truly well looked after. As I said they are both top blokes and I am sure will be able to help or advise.