If you don't have anything and are short of experience it would be very hard not to get a complete motor from this guy.
http://www.bigalstoybox.com/

Consider that you will have about $5k in the blower,carbs , linkage,fuel lines and scoop regardless of who you get it from. Another $5 in a good long block and the rest of the stuff.

If you just want a top kit he can do that too. You will get stuff that will work without a lot of effort.

I did my own but considering that I've done blown applications since the early 60's I knew what I needed. Cost wise I bought new rather than used junk.

The result is a very, very street friendly machine that get livable mileage and simply starts with a single pump of the gas and hit the switch. Idles at 750 and can sit in traffic all day long.

I made 14 laps at back to the 50's and hardly stepped on the gas at all. Each took about 30-45 min. Never got over 205 deg.