alright ...here's chapter 1 ...answer to your first question :
1) have a solid guess at a model based on it's body characteristics
Alright ...so lets focus specifically on Vespas. Vespas are divided into 3 different categories, each with its own engine topology which is INCOMPATIBLE with the other categories : Smallframe, Largeframe, and Widebody / wideframe.
Widebody's came first, beginning in 1946 and the last surviving production widebody probably being the 1961 GS150. These are called "widebody's" because they have a really wide swingarm on the engine.....and the cowls on them stick out REALLY far, giving them a very rounded tail. The most identifiable characteristic to me as far as whether a scooter is a "widebody" or not is whether it has a door underneath where the fuel lever / choke knob tends to be ( 6-10" below the seat, forward facing on the frame ). This door almost always allows access to the carburetor of the engine ( though not always - the GS150's carb is mounted on the engine itself ). Any of the old fenderlight vespas ( faro basso ), Rod models, 'handlebar bikes' meaning they had tubular metal handlebars rather than a substantial cast headset like most later bikes, any Vespa Allstate up to and including 1957 ... these were all widebody's. They were all 8" wheeled bikes with the exception of the GS150 and GL150, which had 10" wheels ( the deluxe models ). On all of these bikes, the engine side cowl was typically detachable, or at least swung up on a hinge to give access to the engine. The other side cowl was a glovebox. ( click here for a pic of a "fenderlight"
http://www.vespaclub.com/tecnica/1954-125.jpg )
Largeframes - these came next, beginning around 1957 ( the VNA being the first i'm aware of ). This was the mainstay of the vespa line, boasting the most amount and variance of models within the category. The smallest displacement largeframe was a 125cc ( the late 50's / early 60's VNB 3-speed 125cc engine, and largest being the P200. It underwent a constant evolution throughout the decades ... the early 60's bikes being rounded like their widebody precursors ( bikes like the VNB, VBB and GS160 ), but the mid 60's marked a switch in styles to a more sleek and angular look ( the SS180, Sprint, Super, etc ). In the late 70's they switched over to a VERY angular, boxy styling called the "P-series" and replaced some of the normally metal components with plastic. All through the eras, there was always a 'sport' or deluxe model ..the GS, the SS, the Rally, the P200... these bikes always had a Glovebox inside the front legshield, rather than the side cowl. The cowl became a cover for the spare tire, and so the cowls had to be sufficiently large enough to hold the 10" tire. Budget largeframes tended to be 125 or 150cc, with 8" wheels... and if you spent more money you got a 10" wheeled model and higher displacement 150cc or whatever the sport bike was boasting (160,180,200 ). The largeframes continued on into the 80's: the P-series, T5 and even the Vespa Cosa are all largeframe vespas and their engines are all compatible with eachother ( with some tweaking in some cases ). A common thing for people who want the styling of a 60's scooter but the power and reliability of a P-series is to install a Pseries or Stella engine into an older largeframe bike like a GL or Sprint. ( here's a pic of a largeframe... in this case, a '63 GL150 :
http://www.vespaclub.com/tecnica/vespa-56-64/1963-150GL-high.jpg )
Smallframes - Starting in 1963 they did a completely different engine topology ...made it much more compact, slimmed down the scooter, and made it smaller and lighter. This was the smallframe ...and rather than having detachable cowls on the sides like the largeframe and widebody, it had a single-piece body with just an access door. ( sometimes just on the engine side, sometimes on both sides depending on the year / model ) It lacked the prominent "hips" of the previous vespa models. This was a more streamlined bike - one that actually completely changed the styling of ALL vespa offerings after it. The release of the smallframe marked the end of the curvaceous 50's and early 60's styling, and heralded in the stylings of the SS180 and Sprint. Smallframe engine displacement tends to be 50cc - 125cc, but they can still be very torquey engines. A 125cc smallframe will run circles around a 125cc largeframe...probably a 150cc largeframe as well. The power to weight ratio is much higher. They are also quite good for tuning, as the engine can withstand more beating than most largeframe engines, and as such the fastest racing vespas tend to be kitted smallframes.
Here's a pic of a smallframe : a primavera (
http://www.vespaclub.com/tecnica/1968-125primavera.jpg )
⚠️ Last edited by Rover Eric on UTC; edited 5 times