Nice job on the body but bodo will not hold up on a mower hood, too many vibes and too much flexing.
Dents have to be knocked out with a hammer & dolly it is a lot easier than it looks if done slowly with light taps.
As this is your first try you get a lot of slack, but you are approaching this project in exactly the wrong order.
You start with the mechanical , get the engine to run, get the mower to run, then you pull it down and do the cosmetics.
Same general proceedure with a mower , motorcycle, or car.
Simple reasons The enine could be totally unrepairable so you have put in all that time & money to find you need a new engine.
Rmoving major parts like engines , clutchs & pulleys usually damage fresh paint work so it has to be touched up or worse redone.
Apart for repairning mowers for a living, I ride & restore vintage motorcycles.
So starting with the engine,
Try to turn it over with plug in.
It should go hard, soft very hard hard then soft again , hard as the valve springs compress then very hard on compression.
This is not very scientific but worth doing to ensure every thing is working.
Remove the head on the engine and press hard on the piston while rotating the engine, it should move up & down evenly with no clicks or binding.
If either of them are felt a full strip down will be needed if the motor is repairable at all.
Check the coil for spark, without the head on you can turn over the engine quite fast so should get a really good and easily visible spark.
You can do all this with the engine in place in fact I find it easier to work on the engines in place however you will have to remove both of the belts from the front pulley
If you have the funds buy a manual for the engine and another for the mower.
The mower manual is available on line from MTD and the manual will be in your local library or get one of the CD's / DVDs all over evilbay.
Even better is the Must have manual 2 available from here
"K&T Parts House Lawn Mower Parts and Chain Saw & Trimmer Parts"
Your motor is called an L head by Briggs & Stratton so you want the L Head manual.From memory it is also available from the same site just work back to the main menu and then go to the Briggs section.
Next pull the deck off, tie some rope around the front axel and hoist the mower up vertically so you can get to the underside realy easily.
Have a good look and get some one else to work all the controls while you are watching to see what each one does.
The speed control on this mower works by moving the centre of a pair of pulleys ( top of the bottom pulley & bottom of the top pulley ) and these wear badly and are very expensive to have a shop replace as it takes a long time to fit & adjusts so the labour costs make it uneconomic to repair, but good for you.
When the transmission starts to play up most owners toss their MTD's when they recover from the shock of the repair bill.
Get this down pat and you will have a source of income rebuilding & selling MTD mowers, there are a lot of them out there and you can pick them up for peanuts ( as you have just found out ). Bad news is you must use genuine MTD belts ( not cheap ) on this system as they are different from standard belts.
Seats are very expensive I replace bad seats with boat seats which ar quite cheap , actually more comfortable and have drain holes so don't get waterlogged.