FWIW when I fixed my Lister VA I replaced the original aluminum piston with a cast iron one and it didn't seem too badly off-balance; it just skated rather gently across the floor.
I realize this is opposite to what you are proposing to do, and it is certainly not running at 850 RPM but hey, nothing ventured nothing gained. There is a lot of info on this forum about engine balancing.
I was unable to free the stuck rings on the original piston, which is why I replaced it.
I have not run the engine very often, but I do have a new aluminum piston for it, when I get a chance to change it.
https://www.youtube.com/watch?v=kYbySo-28pIAs you can see in the video, my son is standing on the wooden skid to help slow it's movement across the floor.
I am still looking for an original fan shroud for it, in case anyone has a lead on one.