It's not just about flow rate. The X-flow gets better results, because it keeps the charge cooler, as the inlet isn't right next to the exhaust manifold. A cooler, more dense charge (fuel + air mixture) means you get a bigger burn = more power Of the counterflow ones, the PB is your best bet on a Mk2 (I think that's what you've got) because it's got bigger inlet valves than the earlier Kjet heads.
I have been trying to get hold of an ABA/X-Flow head and block for ages with no luck I was lead to believe that the K-Jet enginesprovided more powerin stock form?
Try the US eBay they should have loads of them, i was speaking to a few US military guys at the weekend who are stationed here in Germany and they were saying they are in abundance in the US due to all the VR6 swaps. If that fails then look for the Mk4 Golf or New Bettle engines as they used the crossflow heads! Or lastly try VWVortex in the classifieds there maybe some for sale or swap as you'll probably have something the US guys want like textured top bumpers for a Mk3 or something!
Try Matt at Designadubs. He had one recently - he was trying to sell it to me when my head let go. KJet and Digi engines have the same 'book' power figure. However, the general consensus is that Kjet gives better throttle response. On the other hand, Digi is a bit more unleaded-friendly, and slightly better on mpg. Digi heads have 40mm inlet valves VS 38mm for Kjet.