Hi CameroninNSW and welcome to the forum.
What you have there is approx a 1980 full crank 160cc 2 stroke. In my opinion they are a nice, well balanced, smooth running machine with ample grunt.
So to answer a few of your questions, the spring in the starter usually chops out because people are too rough, they release the cord after startup rather than guiding it back in. The removal of the decompressor places fatigue on the other starter parts; rope, gear, etc but the spring usually remains unaffected as it's sole purpose is to rewind the rope after start up.
You are spot on about the vacuum lines and why people do things is something nobody can answer. My concern would be the zip tie holding the diaphragm cap in place and whether it seals correctly thus allowing the governor to work as it should. if there is a vacuum leak from the cap the engine will over rev and the consequences would be dire.
Some parts are interchangeable, carbies, pistons(one way only, form newer models into the older models and not vice versa) and thats about it. This is a full crank engine and from 1982 onwards the "newly improved" powertorque engine replaced this which is a half crank motor.
Prior to getting into this one too deep it would be wortwhile to remove the muffler and do a visual inspection of the piston and rings and examine them for scoring or ceased rings.
Do your thing and let us know how you get along.
