I currently use new AMC heads on my rebuilds for the following reasons 
the type 4-2 heads take a beating and see hotter temps than most other air cooled heads with the exception of the zambonis ice resurfacers that use a type 1 single port fueled with propane. I have stopped tiring to rebuild OEM heads due to the fact that most cores are just plain old used up. The AMC heads have gotten a bad name because they have in the past dropped seats like the OEM heads did. The current version of the air-cooled AMC are staked in place and I have not seen any of these newer staked heads loose seats or have any issues with seats that have gone soft (pounded out).  The AMC heads are made of a different alloy that is much less likely to crack but the new alloy also tends hold the heat a little more so they do run hotter. 
Aircooled type 2-4 engines need all the help they can get to keep the heads and pistons as cool as possible. In my rebuilds I currently use a cam with higher valve lift but not more duration so as not to affect the injection system. I hone the cylinders for more piston to skirt clearance as well as notch the con rods to spray oil at the pistons. 
the only other options I offer is a 2mm larger valves 42 intake and 36 ex with the extra hard seats and a high flow valve job nices upgrade 

Bob Donalds 
Boston Engine 
http://www.bostonengine.com