To retain stock heater boxes on a 1600 based engine you have to stay with the stock (35x32 mm) valve sizes. Stock heads are only capable of 85 or so hp so a 1915 cc engine that's set up to rev to 5,000 rpm (Engle W100 cam, 34 Ict dual carburetors, 1 3/8" exhaust, 8¼- 8½:1 compression) would be about the limit. You could build it larger but the engine will run out of breath (and not really rev any higher) at a correspondingly lower rpm. I have never built an engine combination like this to try it, but my experience tells me this is about the limit with stock heater boxes.
That said, this engine in almost any Speedster would make a fun, zippy around town car (especially with a 4.37 or 4.12 ring & pinion equipped transaxle). I'm going to pose this question to a friend who is much more knowledgeable than me- more later. Al
PS- I'm assuming by heater you're talking about the stock heater boxes- please correct me if I'm wrong...