If you are starting from scratch you better budget at least 2K. To do a 4mil you are going to need a crank, pistons, a cool head (with domes cut for a 4 mil) and porting to make it run right. Thats a minimum. To do it right you may as well put a good clutch in it or you will be soon. More than likely your stock clutch basket is beat to shit so get a billet basket and replace the pressure plate and inner hub. If you dont do all that, you will be on here after you ride it the first time asking why your bike creeps in gear when the clutch is pulled in. By the time you buy gaskets o rings sealer etc etc you are gonna chew up a good portion of that 2k.
Now Since you are in there get a pancake bearing, mod the shift star and the shift shaft (replace that fucker if its worn). Take a look at the impellar they are known for being fucky. Also what kind of pipes do you have? Reeds? Carbs. Next thing you know you got 4 grand in this turd.
If you are on a tight budget, stay with the stock crank. Get it welded and trued. Keep your stock cylinders, have them bored to clean them up and put some new pistons and wrist pin bearings in it. Seal that bitch back up and ride it.