You can surely (no I am not caling you Shirley ) try it, but if the decomp pin isn't gonna be the right length, your e-start is not going to work.
Shirley, dear, he doesn't have an e-start; it's a 426.
You cannot use both cams from a 450 in the 426 because you would need to use a degree wheel to correctly set the cam timing, since the marks on the 450 cams will not align with any part of the 426 head once the timing is set. Adding a YZ450 cam of any year requires that the intake cam be properly aligned and then the exhaust cam gets timed relative to that. The Hot Cams auto decomp exhaust cam for the 426 is made for that engine and times the same as the stock cam.
YZ450 cams come in progressively more aggressive flavors: '30-'05, '06, '07, and '08-'09. The '03-'05 has the most grunt, the '06 is a nice balance of low end and top end, whereas the '07 or later cams are apt to be too much biased toward top end to be to your liking.