milling the head will not give you more speed. more torque yes, and yeah do it.
if you have a 20/25mph model (look at the sticker) then get the bigger header. if you have the 30 it can hang.
assuming you have the M48 engine, a dellorto 13:13 goes right on and is your best bet without a larger intake. also drilling out your stock intake to 13 or so is worthwhile if you can do it straight.
you can also raise your exhaust port a mm or two and widen it a little to gain some speed, should be able to hit 35 pretty easy- fourty not so much...