My advice is for you to talk to a local machine shop. Many people go with aftermarket small block chevrolet (i.e. SBC) valves and guides. They are cheap and high quality. Many people here use Manley stainless steel valves, which you can buy individually from Summit Racing. Most any decent shop can cut the seats for the bigger valves and install the new guides for the valves and hardened seats for the exhaust valves. You will need help with valve springs, seals, and retainers but perhaps Bob or someone (including your machine shop if they're decent) can chime in for specific details on which springs can be recommended for certain valve configurations such as the 1.72" intake and 1.5" exhaust valves in the examples in this thread. Don't forget your cam specifics either. Schneider Cams is one place that will regrind your cam to new higher performance specifications (one grind they have is called the 290H, for Hydraulic lifters, and is said to be a nice profile for performance). There are MANY options for this. Have your lifters reconditioned at the same time. Cams and lifters are available from Opel GT Source as well. You may also want to consider taking the porting details mentioned here to your machinist and see if he will do it for you. None of this stuff is impossible...just take the time to read and learn what should be done and then go and find someone to do it. There are part numbers posted in earlier entries in this thread. I copied a link below for you as an example of the manley valves you might consider:
http://store.summitracing.com/partde...5&autoview=sku might work for your intake valves. (It's really an exhaust valve, but it would work for an intake valve in your case since it is the right size).
http://store.summitracing.com/partde...5&autoview=sku might work for your exhaust valves.
You would then need guides installed (ask your shop), and once you decide on springs, you could have any necessary machining for new spring seats completed. Your shop may also have recommendations (or perhaps someone here will?) for seals and retainers.
It's a bit overwhelming when you first look into it, but once you read and ask the right questions you'll arm yourself with the knowledge you need to get a nice, sound, high performance engine.
Search the archives...nearly all the information you need is likely already here somewhere.
Todd