There are numerous threads on here about making modifications to the 128 and the X, both engines are effectively the same with some minor dimensional differences that won’t affect an install. For the most part your quest is just changing parts, it requires little in terms of actual modifications to the car. I doubt there are many videos on how to do this as most of these cars were modified before its existence or the current modders are older, know how to do it and likely don’t care to be videographers
An engine from a 1979 or later X1/9 or Strada will fit just fine in place of whatever engine you already have. A fuel injected engine can be easily changed over to a single or dual carb set up (including using your existing intake manifold and either the carb you have or a replacement carb.
I would suggest spending a number of hours reading through the Front Wheel Drive part of the forum (and the X part of the forum for good measure), a member recently changed his car from a 1300 to a 1500. If just changing the engine there are parts which need to be changed over to have the starter work properly with the 4 speed bell housing, this is delineated in that thread. Changing a 128 to a five speed has two different methods, the Yugo based way and the Strada way (There is a third option which is a 4 speed to 5 speed transmission modification which was rare and expensive back then and is even more rare now).
Reading a 128 repair manual will give you all the information you need to pull the engine, the Haynes manual is cheap and good enough for that and easily acquired. This is the factory manual:
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The Clymer manual (also good):
THIS BOOK IS IN PREVIOUSLY USED CONDITION. GOOD LUCK!
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Personally I would get the car running as is to see where you are on the rest of the car first before getting deep into modifying anything. This way you can see what the fuel system is like, electrical issue and so on. The manual will help you with all this. I would also download the wiring diagram which can be had from the MiraFiori.com site by becoming a member of that forum.
Again, reading through this forum for a few evenings will give you a great deal of information about any changes needed to swap another Fiat engine into yours.
In regards to the adapters it is normal to go the other way, choose a wheel and then an adapter for the offset and bolt circle of the wheel. Adding 19mm of offset to a front wheel drive wheel on a front wheel drive car is unlikely to get to a geometry that would be appropriate. I would find the wheels you want or need and then decide how to mount them, if a typical front wheel drive wheel at 4x100 you likely just need wobble bolts/nuts and hub centering rings to ensure they stay concentric. Sometimes the wheel center hole has to be machined to match if you acquired a wheel with a smaller diameter opening.
As for go fast parts the 128 in its various forms and the Yugo are still popular cars in Europe, particularly eastern Europe where there are a number of vendors who make some amazing engine parts to make them go faster. I will see if I can scare up a couple of links.