So you got all the easy stuff off, huh?
Your descriptions are a bit vague, which is part of the problem. You say the fork won't come out, but you don't describe if it's locked solid, or steers but won't come out, nor do you say what hardware, if any, has been removed.
As for the overhaul, you may have all the parts, but you don't have the tools. Taking the wheel off doesn't get you into the hub bearings if you need to grease those, and that's where those thin wrenches come in. By "fork stuck in the steering tube" do you mean the hardware is off but the fork still doesn't move? If it steers but doesn't come out then there's still hardware to remove. If it doesn't move at all, like won't steer, then get another bike. That level of corrosion could mean the frame is useless. And again, if you've got severe wear on the front sprocket teeth, the rears are probably no good either. New chain and sprockets all around. Can you get the freewheel off the rear hub? Not without the right tool and a bench vise. Put the freewheel tool in the vise, set the wheel on it, and use the wheel itself as your wrench, turning counter-clockwise. The freewheel will be VERY tight, because every time you press a pedal it tightens against it. I've seen guys use the freewheel tool with a crescent wrench and a hammer, but that's a good way to break the tool or the freewheel, and still have a junk freewheel on the hub. And if you want to replace individual sprockets as opposed to the entire freewheel, that's yet another tool, usually a chain wrench that you use on the outermost sprocket to unthread it from the freewheel. Once it's off, the others just slide off. Again, that part will be TIGHT!!!
As for the crank, if it's like many, there's a nut at each end of the crank axle, under a cover on the crank arm, which holds the crank arm to the axle. Remove that nut and the arm may come off, or you may need a puller to remove it. Don't hammer it, even with a soft hammer, you'll dimple the bearings. Once you have a bare crank axle, one side will have a ring that unthreads and then the axle and its bearings come out. This is another part that has to be RIGHT when reassembled, not so loose it has excessive play, but not so tight it won't turn. Also once the crank arm is off, you'll probably find the hardware that attaches the front sprockets. If it's all one piece, i.e. a cheapie, then you have no choice but to replace all of it. A cheap bike will have both cranks and the axle forged as one piece (with the sprockets riveted or otherwise permanently attached,) which comes out by removing the bearing on one side and sliding the whole thing out after removing the pedals. On a bike with good parts the sprockets bolt to the spokes of the crank wheel. On my old Schwinn 10-speed, the small front sprocket is removable, five bolts, but the large one is not. You probably have three front sprockets on a mountain bike. They may be all 3 removable, or just 2, or maybe it's all one permanent assembly and has to be replaced as a unit.
