you can, I have done a few myself years ago, but these days the price of parts is nothing short of piracy and the price of new mowers by ratio and wages has tumbled. I would not bother getting my hands dirty with it. Do not get it done in a repair shop unless you like to cry on your bills
Replace the engine...End of story.
That largely depends on the quality of the engine. If its a cheap/old one its junk. Most often a cheap engine is fatally damaged by a seize. However... The two main culprits are heat and lack of lubrication. So, allowing the engine to cool and making sure its properly lubed can sometime un-stick a mildly seized engine. (always check a small engine's oil level before each use) If it is jammed a bit harder spraying a penetrating lubricant (like PB Blaster) into the spark plug hole can help. Severely seized engines would need to be taken apart and rebuilt.
You need to overhaul the engine. Your first mission is troubleshooting: did the engine seize because the pistons seized, or because the main bearings seized? To do so, pull the heads and try to remove the cylinders. If they won't come off, piston seize. If they will and when you have them off the engine won't turn over...bearing seize.
clutch for (your model mower) on eBay or other site that sells mower parts
Take it to Sears.
to fix that so it vibrates more throw in a 454 big block Chevy engine you will be guarenteed to have your teeth jarred out
Yes .
That depends entirely on what's wrong with it. If you have no knowledge of this fairly complicated machine, you should take it to a 'small engine and lawn mower' repair shop.
If you've pulled the plugs and the cylinders are full of gas, the motor is seized, won't turn at all (have you put a socket & bar on the crank?) then replace the motor with a used/rebuilt one would be the easiest.
You can attempt to fix it by reading this http://www.lawnmowerman.tv/Troubleshooting.asp Here its basically a homemade manual about lawn mowers and how to fix them. They give you helpful tips and the types of oils,blades that your mower needs.
Depending on the engine model, it can be very expensive. If it locked up, and won't turn over when you try to start it, then the engine is seized. You may consider buying a used engine from a salvage yard, or just sell the car.
Many times it is better to replace a starter than to repair it unless the repairs are minor. A lawn mower engine repair manual would be the best guide. You can find one at your local public library.