Before thinking about buying parts, get the motor fully stripped and see what's wrong with it, and exactly what its going to need.

If the cylinder is not badly corroded, it might not need a rebore, and even the piston maybe reusable?

The piston wont come out easily, as the rings will be rusted to the cylinder wall. Might be very difficult to get it out without damage, but if the corrosion is not too bad, heating the piston with a propane torch, squirting penetrating fluid round the crown, and then trying to get it moving with a piece of wood and a copper hammer, will sometimes work.

Try the same thing a few times if it doesn't work the first time, but you will need a pretty heavy-duty torch to get the piston hot enough. The purpose of doing this is to try and break the bond between rust on the rings, and on the cylinder wall.

There are really bad parts out there. Again, check with Enduronut on piston if you need one.

You'll need crank main bearings and seals too. AllBalls makes a kit with OEM quality parts... used to be about $25 but now more like $40. (Still MUCH cheaper than Yamaha though) Part number 24-1073. Will fit the 175's from 1969-81 and includes the better true SW-type dual lip ignition-side crank seal to seal vacuum and pressure. (Many Yamaha 125's and 175's only had a single lip seal on ignition side... YZ and IT's had the better SW type)

Mr. Crankshaft is another place for crank work. Rod kits are super easy to find for the 125/175 and he used to stock them. I think it was $175 for a crank rebuild including parts last time I did one. Probably higher now.

Anyway, probably looking at around $400 to $500 in parts, machine work, and shipping to rebuild the crank and bore the cylinder for a new piston (if needed). Labor to R&R the parts not included.

A bargain for such a fun bike. Well worth the effort and expense.

Thank you guys for the help! Learning alot from this.

Well its not "good" haha.

God bless Kroil baby. Took some wallops to get it out but here it is. Looks like all it needs is everything!

Crank does spin, but yea..

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It’s really a cool bike if you’re on a budget do you have the seat, airbox what does the inside of the gas tank look like I always try to figure how much I’ll have in a bike but you got for free so that’s a bonus. I’m definitely not trying to discourage you but I do think it would be a fun bike 

The piston looks as though it can re-used, and maybe even the crank? I would suggest stripping the motor completely (you will need a flywheel puller, and ideally an electric or air impact gun), as you will need to replace the main bearings, and when its stripped check the crank. Try and clean up the cylinder, and check if it has deep pitting , rather than surface rust. If it has deep corrosion, you will need a new piston and a rebore.

Inside of the tank is pretty nice...which is mostly why I was so hopeful for it.

I do have the airbox, but unfortunately no seat.

Ill get the rest of it tore down and post pics. Everything turns nice, nothing seems tore up.

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As long as you guys dont mind putting up with me, I think this belle has one more ball in her!

Cleans up nice. No frame damage or spoke damage at least

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Im gonna try to get the rest tore apart today. Anything I should look out for or be careful with?

I assume the flywheel stator pull of one side. The clutch and all the gears come out the other side....then just split the cases?

Don't know your experience. 

Use the correct flywheel puller.  NOT a jaw puller.

Do not try to use the outer clutch hub to hold the inner hub...  you will almost certainly break the outer hub.  Air or electric impact generally will zip the crank nut and inner hub nut loose.  Or use a proper tool to hold inner clutch hub.  See manuals

Crank will need to press out of the flywheel side case.  A simple two bolt puller (often rented free at auto parts stores) will work great to push the crank out.  There are two 8mm threaded holes under the flywheel for this purpose.  See manuals.  Hammering is not good for the crank and even with the nut threaded on to protect the threads, often the end of crank is damaged.

Awesome thank you! Im an inexperienced and learning, so advice like this is great.

I have
tusk 24/27mm flywheel puller
tusk clutch vice grip holder
M18 big boy impact
Basic pullers/jaws

Noted about not hammering on the crank.

Due to the rust, you might need to press crank from clutch side too... though normally a slip fit once nut, gear and spacer are removed. There are two 8mm threaded holes in case on that side for a two bolt "steering wheel" type puller... one of the holes probably has a seal retainer and other hole will be straight across from it.