Hi. Welcome to the forum. You may want to post a "hello" in the new members section telling us a little about you. People tend to get better replies to their posts if they do. Now to the questions;
Q) I noticed the fuel reserve on the bike was 'on'. Should this be switched to 'off'? I assume so but want to check. What happens if you ride with the reserve on? The tank is about 3/4 full at the moment.
A) Does the tap have 3 positions - off, on, reserve)? The tap should be on, "on" and as the fuel level goes down the bike will splutter and eventually stop. Then switch to reserve to get you to the nearest petrol station, remembering to turn the tap back to the on position when full. If you ride on reserve and run out of fuel, you are stuck! Switch to off when you park up. Experienced riders can tell by a bike spluttering when to switch to reserve without pulling over.
Q) Another thing. The bike has a crack on the front around where the front head light is. It's not a real problem just cosmetic, but is it possible to buy a new piece for this, I would like to if possible?
A) providing it is not in the glass, then it should still be legal. That said, most of the wiring connections are behind the headlight so any water ingress will eventually damage these and then you will have gremlins. The best place to get a replacement is off ebay. These are still popular bikes and there are lots around.
Good luck.