Pop the cap and you'll see a large nut. Loosen it, do NOT take it completely off (top of nut even to top of shaft, you'll use it later). Rock the steering wheel while pulling on it to see if it'll work off the shaft. There's a cotter key like a half moon shape that is in a slot on the shaft and steering wheel. IF its really stubborn, spray liberal amounts of WD40 or other fluid that (name brands escape me at the moment) will break up rust, etc. Let it sit for 30 minutes then come at it again. If it still doesn't break free of the shaft (up to the nut), take a hammer (dead blow hammer is best) and rap the shaft/nut (nut to even the top of the shaft) a couple of times while you're pulling on the wheel with your knees and one hand.
Once the steering wheel pops up to the nut, you can then remove the nut and steering wheel but make SURE you don't lose the half moon shaped cotter key. Its likely to be stuck to the steering wheel housing or the shaft. Clean everything while you're inspecting the components and grease the steering wheel shaft including the cotter key (once its cleaned up).
Yes it can be replaced with an aftermarket steering wheel. Vast majority of them are tapered shafts and you'll know that as soon as you see yours. Getting the wheel (or new wheel) can be tricky as you'll have to align the slots with the cotter key in and its really easy to dislodge that key during assembly. Just be patient and in the case, grease will hold that key to some degree. You'll find that cocking the key downward (towards you) during reassembly will help that wheel slide on. Don't worry, it will level that key out to make good contact to make it turn.
Most wheels I've dealt with have 3 slots, so its easy to position it like you want it. Run into any problems, just holler.