Here's what I recommend...

1.  Slip the retainer and felt onto the shaft first, as the clutch splines are smaller  than the transmission end. Slide the shaft in, then slide the retainer and felt into place and secure. Then take the coupler and put it on the shaft. You should be able to slide it on all the way, so that none of it is sticking past the end of the clutch shaft. Then once you have the motor in the frame, just slide the coupler back onto the transmission input shaft, making sure to line up the hole that the bolt goes into. The two longer bolts that clamp down can then be put in. They need to be the right length, and it is better to use the star type lock washers, as the clearance between them and the belt pulley drive can be tight. The thickness of a regular lock washer can set the nut out enough farther to make it tick against the belt pulley housing.

2. The style of the thermostats has been changed and improved. They can be a little confusing as to how to put them in correctly, but they work fine. Unless your going to be working this tractor hard, I'd say the 180 degree thermostat is going to be better. The engine burns fuel best at working temperature, and if it mostly going to be in parades and doing light work, the 180 degree one will keep unburnt fuel from building up in the oil.

3.  You can add a valve by draining the oil, and removing the top cover and the reservoir body from the base plate. If there isn't a valve in there, there will be a cast iron plate that covers the spot where it goes. All you have to do is remove the plate and replace it with a valve (and put in new o-rings while you are there). Put the reservoir and top back on, add your handle, and you should be all set. 

You might want to take the cover off for the lever that is missing. I have come across units that were dual remote, but for whatever reason a second lever was missing. If there is no oil in the unit, you'll be able to see if there is a valve in there or not. 

I would think that Larry Harsin would have an extra valve in everything he has. If not he could probably point you in the right direction.