will any of these bikes work?

All 3 look like they will work. The Schwinn looks like a nicer one but I don't know how much its worth. Try to find out year it was made and if it was made in USA.

You'd have an easy time with the Schwinn, but I am not sure it's worth 90. The Huffy's have a sharper angle in the frame and look like they may be a little tight, but would likely work. For 20 bucks it's worth a shot.

I would try to offer around 70-75 on the schwinn and see if they go for it.

I'd go for that Schwinn! That would be a nice and sharp looking motorized bicycle

Somebody correct me if I am wrong, but chromoly steel is a steel/chromium alloy. I don't know too much about it but I have heard it is stronger than regular steel.

You are correct sir! Chromium and molybdenum.
Hard to drill too...

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.

That is a great bike for conversion providing that it's steel and not aluminum. I'd be pleased to own it