I would like to save some money and get a used 21" rim, hub, and maybe spokes and relace my front wheel myself. Will any 21" rim work? I assume it will need to be a Savage hub. Are used spokes safe? Is there only one length? If there are any other model bikes out there with parts I can use, please list them for me.

Finding just the right spokes would be a shot in the dark. You can measure the length of the stock spokes, then add 1" for the circumference, and you'd have a good guess. The real length will probably be longer because the spokes come into the wheel rim at an angle.
I don't know if you can calculate for that, but you might be able to draw it out on paper?
Your wheel rim will have to have 36 holes if you plan to use the stock hub. When looking for spokes, remember to check the diameter of he spoke nipples to be sure they work with the holes in your rim.

I've been working at converting my front wheel to an 18" and I have definitely found it more complex than I had thought....
Good Luck!!

Most dirtbikes front rims are 1.85 in width. The savage stock I believe is 2.15 wide. Keep that in mind for how it will end up looking.

If you want to save money you will have to sacrifice time probably. Id get the hub and a 36 hold rim, set it up and measure how long the spokes you need will be.  Used spokes are not likely going to work. Most people removing spokes by cutting them out with bolt cutters and throw them away. there is almost never a complete set for sale and usually the nipples are stripped a bit.

You can order spokes that are decent quality from ebay and some other places that come from Thailand. The shipping turnaround is like 3 or 4 weeks, but Ive used them before and they are decent quality. As good as stock.  Those ones you can get for around 30 to 50 bucks for a set.  Id trust those more than used ones. They sell them in several pre-made lengths.

Also, you need to keep in mind that you need a spoke that has an inner and outer spoke. One is bent at about a 90 deg angle and the other a bit more. Sometimes one will be longer than the other also.  dirt bike spokes wont work, they are mostly straight.

Ive monkey'd with rims a spokes a bit and if you want some links to good deals or whatever send me a PM and Ill forward info to you.