I believe a 2007 cc displacement (90.5 cylinders and 78 mm stroke) is as bulletproof as they come. You’re getting as much displacement as possible without weakening the case and keeping more meat in it and the cylinder walls. That coupled with a nice cam and ported big valve heads with nice 40 dual throat carbs and nice exhaust should get you where you want to be.
The 1915 cc in my car works well but not as well as I thought it would work after what I put into it. When the motor was hardly used I did a top end job that consisted on installing big valve CB Performance 044 heads that were ported along with a set of dual Dellorto 40’s, 1.25 ratio rockers, chromoly pushrods and rigid rocker shafts and swivel feet adjusters as well as breather kit and nice exhaust. Many of those parts came from Gene Berg. My big mistake was not splitting the case to install a nice cam and counterweighted crank. I resisted that being that the motor was so low miles; I didn’t want to touch it. Also I didn’t switch the flywheel for a lightened unit. I am seriously thinking about doing all that now in order to try to maximize what I have within that displacement. My dream engine would be a 2007 cc but I would have to incur in a lot more money; (basically switching the whole thing without reusing much of what I have). I’ve heard of great power coming out of 1915s; but then again it’s all down to the combination of components that you put into it; the cam being one of the most crucial ones. The cam really wakes up an otherwise meh engine. I am now thinking about using what I have and swapping the crank for a counterweighted one, as well as the cam and lifters for an Engle 110 type, put in a lightened flywheel and reusing everything else.