You might be surprised how many large old engines are still around, though most I know of are steam.
For an excellent example of surviving gas engines, there is a link on this thread to show newly taken photos of the BIG gas blowing engines at Bethlehem. They last ran in 1995.
In the UK there is a 12,000 hp (from memory) rolling mill engine at Kelham Island, still run on steam, albeit just playing nowadays!
In Australia a 16,000 hp Galloway rolling mill engine has survived.
In the late 1980's there was still a steam driven Mesta rolling mill engine at Republic Steel in the USA rated at around 35,000 hp, not sure if it has survived. There are a few other big US (steam) rolling mill and blowing engine survivors.
In Germany, I believe there are big engines in a steel mill under preservation at Volklingen. I hope there are some gas engine survivors there, they certainly have many steam engines.
Kempton is a must see, one engine running under steam. Now those are very impressive engines! Unfortunately they ran a main bearing and couldn't steam when I was there...but OK again now I believe.
I don't know much about the Cockerill Works Museum at Seraing, Belgium, they had an 1826 beam engine there, they may have info about any surviving gas monsters. (If the museum is still there).