if i were you, and i am in your position, im sooo newbie dude, ive been into this for less than a month ..
but here is what i think is best,,, look for parts online, not to order anything, but simply looking at the parts describtion will teach you alot, and also while u do look for parts make sure you find matching parts ...
once you get ur wheels and tires look for hubs, spindles to atleast be more secured that u wont have a problem findin those parts later ...
put your engine and seat and wheels on the floor from there you can have an idea of what u want ur frame to look like
p.s. since/IF you got your welder, all you can do is practice, practice practice ... on anything, get a feel for it