Just bought a new (to me) GZ250

[Orphanclone]

Just figured I'd introduce myself, my bike, and maybe pose a few questions.

I am learning how to ride motorcycles and I just recently purchased my first bike, a black 2002 GZ250. I only spent $900 on it, which I'm pretty sure is a good deal, but the bike itself has a few..."character defects" we'll call them. The previous (and to my knowledge, only) owner left the bike outside all the time and was none too kind to it, and it shows: the front seat is patched with Gorilla Tape, the fuel tank has a few dings, the left frame cover is missing, the speedometer cable is cracked (and non-functioning), one of the turn signals is also a fluid reservoir, the front fender is cracked, sprockets look good, but the drive chain looks "aged", and there a few non-essential bolts missing from various places on the bike. Most of these things I can live with, repair myself, or acquire replacements from BikeBandit.com or my local Suzuki dealership.

I'm more concerned with the things that I should look for that I don't know about. For example, it may not strike some people as necessary to change the oil & filter or to clean the air filter or to check and possibly replace the spark plug or to check the brake pads/rotors. What is it that first-time bike owners generally overlook?

There are also two mechanical problems that I need some assistance with. The front tire looks almost new, but the rear tire tread is not acceptable by any standard. I know that the 130/90-15 are not common, but does anyone know of a good place to buy them or manufacturers to stay away from? The other problem is a crack in the viewing window of the front master cylinder. I know this piece cannot be acquired separately from the master cylinder assembly (59600-0AEA0), but it there any place I can get a new (or gently used) one for less than $85-90?

Thanks in advance for taking the time to read my somewhat lengthy post. If there's anything else I can do, I'd love to contribute.