Just some tips about the bike you choose:
Road and mountain bikes already have busy handlebars with levers and shift controllers on both sides and you need to add 1 more to each side.
You can either try to just shove the clutch and throttle on as well but besides looking pretty dumb they make it a lot less safe to operate.
The proper way in my book
anyway is to do it is the following:
Completely remove the front derailleur shift control and both brake levers.
Put a dual pull brake lever on the right with the throttle.
Move the rear shift controller to the left with the clutch.
To do this the bike needs a rotary rear shift control you can move to the left so don't buy a bike with lever shifters, especially if the brake and shifter are one piece.
You will need to buy a dual-pull brake lever, $13 at sickbikeparts.com so you might as well get an NGK Iridium plug as well so figure ~$28 delivered.
In my opinion
the safest and easiest to operate controls on a bike with dual shift and brake controls on a 2-stroke gas build ends up looking like this.
On a 4-stroke builds with no clutch lever or electrics this of course is irrelevant like this electric built I am just wrapping up, I left her handlebars intact all but removing the right grip and putting on the half grip rotary throttle and a 1/2 grip on the right.
Other than making sure your used bikes back wheel and brakes are sound that should steer you in the right direction to buy a bike you want to put a 2-stroke on.