Visit your local junkyard, auto auction, or dirt bike shop to purchase an old bike to rebuild and restore. The type of bike you choose to rebuild is completely up to you and your budget. Prices vary depending on the name brand of the bike so keep that in mind when making your final choice.
Strip down the engine. Sketch a rough drawing of the crankcase side and when you remove the bolts from the crankcase draw them in the appropriate places in the sketch so you will know right where they go when you rebuild the engine.
Split the case of the engine by taking a spanner and placing its middle on the end of the crank. Tighten up the spanner and use a soft mallet to help pry apart the casing.
Take apart the rest of the engine piece by piece and clean all the parts individually. Use a degreaser product to clean all the nasty buildup of grease and other gunk off your engine parts.
Use rust remover to help get rid of any rust found on any of the bike's parts. There are a wide variety of rust removers on the market so shop around for the strongest cleaner at the most affordable price.
Take paint remover to the frame of your bike to remove all the old paint from the frame. After you have all the old paint removed you should take some anti-rust primer and coat the frame with it so as to prevent future rusting.
Paint the bike frame using your favorite color of paint. You can paint your bike with a brush or a spray can of paint, but it is a lot easier if you purchase a spray gun and spray on the paint this way. The spray gun will give you better coverage and make the process go a lot faster.
Remove the old seat cover and replace it with a new one. Remove the metal plates at the back of the seat, pull out all the staples and take off the cover. Put your new cover on, staple it in place, and bolt the plates back on to the seat.
Replace any damaged parts in the engine. You will more than likely have to replace the bearings in the crankcase, as they may have rust damage. If there are no damaged parts then move on to the next step.
Put the engine back together and bolt it back on the bike. Basically, just work backwards from the process you used to take apart the engine and make sure you put the right length bolts in the right places.
Test-drive your newly rebuilt dirt bike. Be sure you wear appropriate safety gear when taking your bike for a ride, such as a helmet and pads.