Modern Vespa

My '64 Allstate has an SI 20/15 installed and came with an extra 20/20. She runs well on the 20/15 and I was wondering if there would be any noticeable improvement gained by rebuilding and installing the 20/20? Or is it such a minor difference that its not worth the time and money? Thoughts? Also, would I need to completely rejet the 20/20?

Twice the area (176mm vs 314mm) in the venturi where the gas and air mix. You will have to rejet some and with a more modern exhaust it will wake the engine up.

The 20/15 is a great little carb but was made strictly for economy. You won't break any speed records, but you'll definitely notice a difference swapping to a 20/20. It'll cost around fifty bucks for a pack of jets and a rebuild kit, and take a maybe couple of hours. Whether it's worth it is up to you.

Sounds like a plan. Does anyone have a starting point for jetting? OEM? Thanks.

AC140 BE5 MJ110 42/140 to start with

Now we await the questions as to what that all that means.

Air Jet - 140
Main Jet - 110
Idle Jet - 42
Atomizer - BE5

Since I will be opening up the breathing on the front end of the engine, I should likely look for a muffler with less backpressure, correct? Any suggestions for a mildly improved (from stock) street muffler for this type of application?

FWIW, I found a OEM jetting chart.

As I mentioned before, it would help to have a less restrictive exhaust. You are on 8 inch wheels, so you may want to check out this thread: Has anyone solved the box exhaust scraping on 8's? +Solved!

Is it worth for 2 ports engine? I've heard that SI 20/20 won't do much on 2 ports

It's worth it.

So, I finally got around to disassembling my spare 20/20. Inside is:

Idle jet - 40-130
Air jet- 140
Atomizer - BE3
Main jet - 94
Choke jet - 60

I am going to order the SIP Road 2.0 muffler from Mercato. Considering my Allstate's engine is otherwise 100% stock, could I get a starting jet stack recommendation? Thanks again for all the help.

I'd go up 10 on the main.

This is what Jack221 recommended.
I have followed his advice in the past and found it to be very good.

Get a couple of different main jets. + and - the 110 starting point.
You will definitely play with these a little bit.

Thanks for the advise.