Surely an expensive project but when you factor rpm and reliability
once all the work is done the non-pushrod designs need more rebuilds.
Overall price with all the work done to a gx setup will be roughly
the same and even save money in the long run not to mention better hp.

Also the space on the graph as jmac points out is fatter and more
responsive on modern muti-valve designs.

I was always interested in the E15 engine compared to the A15
with the same factory 80hp the E15 is so much cleaner in response,
economical, quieter etc. I prefer the simplicity of pushrods but
when you compare it to a belt driven or chain driven multi-valve
of the same capacity you can certainly notice the improvement.