You're right there!
Especially the Honda 4-strokes are just about immortal. I own a 1974 Honda Supersport SS50, runs like new, a 1972 Honda PF50, runs almost like new (it needs some tuning), a 1963 Honda C310a wich I'm restoring, a Honda Benley CD50 wich I'm building up from scratch and tomorrow I'm picking up my 5th Honda ... another C310a with an extra engine.
The pushrod type engines (PF50, C310 and also your PC50) are literally indestructable, but their poweroutput is low, don't expect them to go faster than 65 Km/h (41 Mph).
The SOHC type engine are a tiny little bit less indestructable but they still live forever and the poweroutput of these things is enormous, I tuned my 49cc Supersport to go 83 Km/h (52 Mph) and still with an awesome acceleration and the sound ... oh man, I love that sound!! And there are still some upgrades I haven't tried yet, like the racing camshaft and a nice Motorsport exhaust wich will add a lot of power my wheeled monster.