Next time you're pulling apart your bike, there's a few things you can do to improve it. If the frame is rusting, you can get some empty beer cans, slice them up the side, and wrap them around the rusted part of the frame. You'll need about 10 to make the patch job thick enough. Then get yerself some hoseclamps or zip-ties and do them up real tight on top of the cans. This saves you all the expense and work of getting the thing welded. Though, if you do have the time, you can use an old car battery and a few small welding rods. Weld the cans thoroughly to ensure a good patch job.
Now that your frame is back in shape, we'll move on to the tires, which, if your bike is like any of mine, are probably flat. First thing you gotta do if the tire is flat is get the thing off. This can be pretty difficult if you don't want to wreck the tire, so I've got a better solution. Your local hardware store probably carries this ingenious product called spray-foam. There's a bunch of different brands, just buy the cheapest (or maybe you can get a partially used one cheap from a neighbour's garbage can). Next, pop the valve stem out of the tire and shove the spray nozzel of the foam in the hole. You're supposed to wear gloves for this but the stuff is really neat to play with so I never do. Spray as much as you can into the tire, trying to fill the entire cavity with foam. If you can't get it to go all the way around, you'll have to cut a hole in the sidewall of the tire and finish the job. Now you're tire will NEVER go flat again!!! You can drive over anything without concern, just like James Bond in Tomorrow Never Dies.
Bikes are a lot like cars, but something most people never think of doing is undercoating their bikes. This is great, because then the paint doesn't get chipped and start to rust the piece of crap into a pile of junk metal. In fact, I like to put undercoating on the entire bike frame. The stuff is cheaper than paint and you can get a nice finish with nothing but a brush. Again, check your neighbourhood garbage cans for leftovers before you purchase any. Your local car body shop might leave some cans out back with enough left in for you to scrape out.
Next, we'll take a look at the exhaust system, if you've got one. Exhaust pipes are not much different than the frame, and you can repair them in a similar fashion, tho for this I prefer to use empty bean cans instead of beer cans, tho really either will function adequeatly. Wrap them around the exhaust leek 10 at a time and do it up with hose clamps. Using zip-ties is also good but you have to replace them more often because they tend to melt.
I've never bought a bike that didn't have a ripped seat. Fortunately, the ultimate tool of man can fix this no problem. Just get out your trusty roll of duct tape, (never, never be without duct tape), and tape the thing up. If you figure it out right, you can even make the repair job look like custom lettering on your seat!
If your fuel tank is rusted and leaking, ride the bike down to your local boat building yard. Once fiberglass is mixed up, the curing process is irreversible, so wait for the boat builders to finish their next job. Then ask them if you can have their leftovers, and take the mixing pot over to your bike before the fiberglass cures. Yank your tank off and dump the gas out. Then pour as much of the resin as you can into the tank and slosh it around, pouring a bit out so you have room for fuel after. You may need to drill out the petcock valve to get the bike running again. Since you ran out of gas you might have to siphon some out of a boat. Incidentally, I've been thinking about using this same technique with the tires instead of spray-foam. If anyone out there gets a chance to try it please let me know.