On any secondhand vehicle you should
change ALL the fluids (oil, water etc) and filters (air, oil). This gives you an idea of how things are.. if you find
black oil - then I'd change it again after some small millage to give it a good flush.
things in oil filters .. they come form somewhere .. best to find out now before the thing stops.
tbg while you are working on the motor .. I'd encourage you to drain and refill with fresh oil the gearbox, final drive and fork oils. Do look for black oils and metal particles ... report back on metal particles .. black oil = ride for a while .. say 500 miles or a month and change again. Oh - check the air filter .. a good shake should free any large bugs out. And take off each carb float bowel in turn and look for dirt and/or water in their bottoms.
If you find lots of things wrong with the motor take a look at a bone yard - another second hand motor may be cheaper than new parts ...