great choice of scoot halflife. I have a 64 special myself so I've been through all the questions that you have raised .
The answer to your questions is it all depends on the condition of your scooter and what you want from it.
All I did was to replace the points and condenser and check TDC and adjust the timing to 19 degrees.
I replaced the coil with a V***a PX200 jobbie which is 10x better than the Lambretta one. I changed the oil and replaced the gear, throttle and choke cables and I replaced the original Dellorto SH/18 carb with a Jetex 22mm carb with a 5889-2 Atomiser and a 106 main jet.
I stripped and cleaned the front hub and replaced the brake shoes and tyres and that is pretty much all I did to mine although I have a 42mm Clubman to fit sometime and I know I need to refurbish the front forks and rear hub.
Once tuned and set it starts 1st or 2nd kick every time and pulls like a train .it will cruise happily at 50mph on part throttle and has done 56mph at which point I bottled out.!
If you need to replace your barrel and piston I'd suggest you consider an Indian 175cc engine kit which you can often find for less than £100
Depending on how bad your gearbox oil seals and rear hub are you might be lucky enough to use it 'as is' but if you are going to replace the engine it would make sense to strip and overhaul your gearbox . You're bound to need new bearings and oil seals and new chain guides and your rear hub may have some play . It would be worth putting a new clutch in and overhaul the gear selector and kickstart ... see how one little thing leads to a complete overhaul
