Generally that Hi/Low range lever is not ment to be moved with the engine running. It can be done but you need to move it from one range to the other quickly or the transmission will spin in neutral. Most people leave it in low which I believe is the forward position.
There are 3 sticks in front of the operator that are the most commonly used. The left stick operates the fwd/rev motion and has a neutral middle position. The other two sticks operate each track. Forward for fast, back for slow and middle for neutral.
There is an engine shutdown cable that should run to the lower left corner of the instrument panel. This cable operates a lever on the fuel distributor on the front/left of the engine. Pulling this cable is the proper way to stop the engine. Generally speaking its not a good idea to cut off the fuel supply and force the engine to run out of fuel. Technically the engine stop lever kills the fuel to the engine but it does so by shutting off fuel flow in the fuel distributor, it does not let the fuel distributor itself run dry.
Your loader is probably equipped with the standard Case 188 inline four cylinder Diesel engine. It's a good engine but like anything else needs regular oil changes. When the 188 "goes" it generally looses the #3 cylinder rod and puts a hole in the block.
There are several filters and a pickup screen that occasionally need cleaning or changing. The first thing you need is the owners/operators manual. It's quite good and gives lots of necessary information. If you plan to do your own work on the machine a set of manuals is indespensible. These are not hard to find but will run between $200 and $300.
Before you run this machine much take time to learn where all the fluids are and check their levels and condition. Running low on fluids or running contaminated fluids is a good way to end up with a heavy expensive piece of yard art.
Best of luck and let me know if I can answer anything else for you.
1968 Case 450