For the throttle, I'm not sure about yours, but all of the later ones, the lever was on the sheet metal that is part of the hood.
Maybe the fellow from the beginning of this thread can help you out with the sheet metal dimensions.
If you can replicate the hood, I know most here would be impressed.
The motor should have model, type, code numbers etched into the shroud - on the top of the motor near the spark plug. Being an older briggs, you will have to look up the code number to find the year of manufacture. Could help date the garden tractor.
Here's my favourite site for info on taking apart a medium flo-jet carburetor:
http://outdoorpowerinfo.com/repairs/briggs_medium_two_piece.asp.
The rear end uses 80/90 weight oil. There should be a fill plug that sticks out at the middle-back. Not sure about yours, there may be a small screw on the back - just over half-way up and on the right of centre. Fill with oil until it comes out the hole.
It's been sitting for 25 years - take your time.
It might not have spark. I would expect that after 25 years. The points should need a good "cleaning". I put that in quotes - because to clean the ones on my first 6hp, I put a piece of emery cloth on a piece of steel and gave each side of the points a good rubbing. Hardest part is that the points are under the flywheel, which is under the shroud.