Welcome to this site! I'm pretty sure there are fitted, replcement seat covers on EBay for around a hundred bucks. My upholstery skills aren't very good...I would have a tough time trying to recover a seat with a sheet of vinyl...a flat seat would be one thing, but all of the curves on a Drifter seat would be a lot more difficult.
I am not a big fan of ski spreaders. Denniskirk.com still offers them (part # 10-20) for about 20 bucks....I assume they are the same as the ones I tried 35 years ago on a 39 HP SnoJet. They are 3/8 inch thick steel plates that extend each ski out about 2".....and they still bent over time, until the skis had a ridiculous splayed look.
If you decide to try this, I think you would have to make them out of 1/2 inch steel.
The other downside is the change in steering geometry. The skis are mounted dead center with the spindles. When you move them out with ski spreaders, the skis are then off-center with the spindle. Whenever a ski encounters an obstruction....a tree root, or one ski cuts down to the dirt...the ski now wants to twist the spindle, which is certainly felt in the handlebars. I think they could be great on a little 250 cc SkiDoo Elan, or something similar. They would certainly add some stability to your Drifter....but I just don't like the feeling in the handlebars when they get yanked one way or the other when one ski encounters some drag.