'64 should have been the 1500, '63 was the changeover year (although the 1200 engine option continued in Europe through '67). Easiest way to tell the difference between a 1200 and 1500 Bus engine (without actually driving one) is the carburetor. The 1500 engine had a governor, the 1200 did not. I guess they felt safe unleashing ALL of those 40 horses at once, whereas the 1500's 50 horses needed some reining in.
I've owned several 1600 T2 Buses, they were no rockets of course, but they could do 65 on level ground without any problems. My '71 Westy had a tach, and at 65 it was about 3800 RPMs. Those boxers (in stock form) don't really spin much faster. 34mm carb won't let them.
Really the Type IV engines, especially the 2.0L, finally gave the poor Bus some decent power. My 2.0L T2 will cruise at 75 all day long on level ground. They don't really like hills, though. But around town, it is actually pretty peppy. But still, the standard issue engine in the T2 in Europe (and other markets) was a 1600 carb engine, as well as the T3 early on.