I would probably call 928 International and ask them. They are the masters of parts and might give you a few minutes time to try and figure this out... I would think most good machine shops would be able to replace valves and check replace "lap" the seals...or whatever you call it to be sure the new valves will seal properly...
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black
I have had heads rebuilt by local machine shops, they did not have any concerns about doing so, and the results were good, so I do not think you need to send them out. The quality of a specialist may be better, and Greg Brown in CA has the reputation of the being the best.
You can source your parts through a number of vendors, including the following as far as I know: 928gt.com 928intl.com 928motorsports.com and 928srus.com