Another question: I am going to have to helicoil at least one of the exhaust/manifold threads. Possibly two.

Should I just do them all?

Everybody likes pics.  The carb was in pretty bad shape (like everything else on this bike - for sure a basket case but we are having a go at it!).

Got the body cleaned up as a start.

Carb body looks great. Did you end up bead blasting it?

Yes, wet blast with glass beads.  And some scraping and brushing.  We will see if the beads clogged passages and ruined it, but there werent many great options and it needed cleaned up.  Will give it a good cleaning before reassemble.

The inside passages we left alone, they look useable where the piston rides - the severe corrosion was in the float bowl.

It did soak for a week in penetrating fluid before it loosened up enough to slide the piston out.  Like everything else, pretty frozen.

wow that carb looks gnarly! it can be hard to blast all that white alum oxide corrosion out of the internal passages!
I'm not an expert on these small bore bikes like some other members here are but here are some parts I would start with
OEM gasket set:https://www.ebay.com/itm/284430004332
or aftermarket:https://www.ebay.com/itm/194827956710
rod kit:https://www.ebay.com/itm/393439094314
main bearing:https://www.ebay.com/itm/373869680989
Main bearing:https://www.ebay.com/itm/373687065597
crank seals:
www.partzilla.com/product/yamaha/93102-25061-00
www.partzilla.com/product/yamaha/93103-28011-00
www.partzilla.com/product/yamaha/93210-180A6-00
Clutch cover seals:
oil pump drive: www.partzilla.com/product/yamaha/93101-10002-00
shifter: www.partzilla.com/product/yamaha/93109-14010-00
kickstarter: www.partzilla.com/product/yamaha/93104-20007-00

You should measure the bore with a caliper first to see what piston to order, but OEM ones are available on ebay or from some members on the forum here.
 

OK, inspecting this carb a little more closely.  The inside passages are not too bad EXCEPT:  The one opposite where the float pin goes.  It is a bit smaller in dia and exactly across from where the float pin mounts - you can see in the prior picture it is full of corrosion.

Scraped a bit more and found what appears to be a brass screw in it - which of course does not come out.  

What is this screw?  (IS it a screw?).  Does it 'have' to come out to clean or adjust or can I leave it alone (sometimes there are just plugs so I dont know)

I think getting it out will not be simple.  Will start with some heat, go from there 

Indeed there is a brass screw at the bottom of this.  Some heat.... nope.  Still soaking, but not sure this one is coming out...

In the spirit that you can never have too many pics, some more progress cleaning up the crankcase.  

And where we are now 

still have the head to do, and havent snapped pics of the cylinder