There's an old thread on here that suggests mowers with stickers such as yours are from the late 70s.
https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/109411/re-sb-45-id-plates.html

If you look on the side of the blower housing (probably near where the carb bolts to the head you should find some words and numbers stamped on there.
It should list "model" "type" and "code" with corresponding numbers below.
Model is the model number of the mower.
Code is a date stamp. Usually written as yymmdd plus a few extra numbers that I believe are related to factory.
So if your engine is original the code will probably start with a number somewhere between 77 to 80.

It's shame what someone did to those handlebars. They're in otherwise pretty good condition and looks unnecessary given it has what is possibly the original engine.

I can't see any obvious cracks in the frame but sometimes you can't see them until you remove the engine to inspect.

It should clean up nicely.