Depends how the engine condition. stock or modified engine.
I only found for 1986 KX500 information. I cant find 1983.
for only 1986 KX500 is
46 horsepower engine.
41 ft lbs of torque engine.
I had experience on my 1983 yamaha yz250. There is big different between stock and modified.
When I modified the bike. ( FMF ported. Mill cylinder head, Kustom Kraft racing pipe. Racing fuel. ) I was raced with it. I said whooooahhhh. That felt like twice power. and more rpms. faster than stock bike. I am so stupid that I sold it. I am very upset and still want it back. Never found it