Here are my thoughts for anyone else who wants to take a stab at doing their first rebuild. First, like Trumpy kept saying to me, don't underestimate the value in using correct, good quality tools. I wasted a lot of time and energy trying to improvise bearing drifts and install the crank using washers and the clutch nut. Even correct specialty tools can be had cheaply enough to be worth buying. A couple of drifts will cost about $20 total. A cheap crank install tool from India cost me $15 on eBay and worked fine, but I had to wait 3 weeks for it. Should have ordered it at the start and it would have been here when I was ready to use it. Other tools I bought just for this project:
Good quality circlip/retaining ring pliers - Channellock 8" and 6.25" ones on Amazon. Do NOT skimp and get the garbage Harbor Freight ones for this.
A set of pin punches. Harbor Freight is fine.
One mid-weight ball peen and mid-weight brass hammer, Harbor Freight.
Crank install tool (eBay from India if you're cheap and have time).
"Fourth hand" cable tensioner tool - there are $20 ones on Amazon although the IceToolz one I bought can't grip the thicker replacement rear brake cable I installed during this project.
Torque wrench with a 10 ft/lb minimum setting (I had one that only went as low as 25 ft/lb, so I bought a cheap-ish Tekton one on Amazon).
Clutch nut tool, clutch holding tool, clutch compressor tool - cheap and good quality from Scooterwest.com
2 sets of feeler gauges that include 0.15mm size - local auto parts store
Propane torch for heating cases
Heavy duty bench vise works great as an engine stand
Shop grease
Zip-lock bags and Post-it notes, or some other equivalent system for bagging and labeling things as you disassemble
Books are OK and can be useful for reference, but nothing beats a good video tutorial for showing you how things should come apart or go back together. I heavily, heavily referenced this video from Scooterwest on rebuilding a P125 motor:
Note that the Scooterwest videos don't show replacing the bearings. Also, as with anything like this, both the Scooterwest and Scooter Techniques videos show "their way" of doing things; you may not necessarily agree with them. For example, the Scooter Techniques video shows three ways to install a crank: with the correct factory tool (or equivalent), with a home made tool, or by just bashing it in with a punch and a hammer! Gah, I would never try that last one. I watched both of these numerous times before and during my rebuild work. Having them copied to a tablet where I could quickly reference them in the garage was a life saver.
By far the most aggravating and stress-inducing part of the project for me was anything to do with bearings. I either removed them incorrectly (tapped out axle bearing without using any heat), installed them too gingerly (main bearing), or just plain didn't realize they were a bearing and just bashed them on with a hammer (clutch bronze bushing). Live and learn, on those. It took me about three and a half months to get it done, but as I said I lost three weeks at a dead stop waiting for the crank puller tool to arrive from India. Another huge time waste was having to sand down the gear shim, because my old (not worn out) gears combined with a new axle, for some reason needed a 1.80mm shim to be in spec for end play. My original shim was a 2.20mm, and the smallest I could source locally was a 2.00. I spent ages wet sanding that stupid thing, when I could have bought a 1.80mm from Beedspeed and waiting a week for it to arrive. That was a stupid waste of time.
Otherwise, I just took things slow and steady, stopped to ask questions on the forum when I got stuck, and tried to work on it a little bit every single day. On weeknights I was out in the garage almost every night after my son went to bed. On weekends I was out there during his afternoon nap. I'm still really proud of myself for getting it done successfully. A coworker and I were chatting about engines the other day and someone else asked me how we know so much about them. I said, "I don't know very much, really," and my friend said, "Uh, you took your Vespa motor apart, down to the *bare cases,* and replaced a ton of stuff and put it all back together, and you didn't have so much as a circlip left over. And it started and ran fine on the first try. You know more than you think." I do now, I guess