I pulled the crank into the left side case. As I was going to install left side into the right case half, the crank with main bearing attached  to it dropped into the right case.
I can pull the left crank/bearing out of the bore by hand just by jiggling it a bit. The right side crank bearing is very secure in its bore.
The left side bore that holds the bearing has no excessive wear or damage and is in good condition.
I think once everything is buttoned up and tightened down it will be fine.  
But I would sure like to have the opinion of some of you guys who have more expertise than I do.
Thanks.
PS - just got new piston, rod kit and bore from Bune.

Crank Bearings should be press fit into case.  Period. There are also inserts that can come loose and sealer "dabs" from those inserts that can cause air leaks.

Crank usually slips into right (clutch side) bearing by hand.  This is not an issue as crank gear nut clamps crank to inner bearing race.

Left (ignition) side of crank is a tight fit into bearing bore and needs to be pulled into place. 

I learned, back when I was about 16, there is no "pressing in" of main or transmission bearings. You freeze the bearings, and heat the cases. Then watch bearings drop in with some oil. If you don't, the bearings chew case material out, and your left with a bearing that can be tugged out by hand. It will walk and further chew out the case. Cases might be toast, unless you know a VERY competent machinist that can aluminum weld and re machine the bearings journals..

Also not an it175 expert, so anticipating blow-back or ignoring.  Like 90% of the stuff i mention here, im a sure-fire way to kill a party.  

Is it likely that a spun crank bearing in the left case could have gotten so hot that it expanded the diameter of the journal just enough that a friction fit or press fit is no longer possible.

Knutco, not sure if your comment was directed at what I said?  Allow me to clarify by saying a "press fit" is a "condition", not a "technique".

For case bearings I agree the installation technique you mentioned is best practice.

 

I was more looking for an explanation, reason or cause that the journal would have been slightly enlarged to the point where the bearing is not a tight fit. There is no apparent physical damage to the surfaces of the metal.

Quidger,  I may not be clear on what you're describing/asking without making assumptions.  For clarity, the "journal" is the shaft...  the part of the crank that goes into the inner race of the bearing. 

The outer race of the bearing presses into the case.  Could the bearing have spun in the case?  Possibly.  The YZ125 and IT175 of that era have soft metal inserts cast into the case that the bearings fit tightly into.  It's not uncommon for those inserts to get damaged and even to come loose...  I will see if I can find any pictures.  I'm not sure why this happens but it seems to me it has to be from heat, load, balance, or crank out of true or something?

Quidger Are you putting in new bearings if so have you double checked the diameter against the the old bearing also you can check the numbers on both bearing to see if they match 

Not towards you markt, it would be the terminology really. I prefer interference fit, but I know press fit is the right terminology. It confused me once upon a time..

I installed new bearings. Numbers and specs exactly the same. 
I am including link to a video that shows the bearings, old & new, being dropped into the case so you get a feel for what I’m trying to describe. Don’t know if this maybe a contributing factor to the looseness in the case but the bike had snapped the rod 20 some years ago and I got it as a rolling frame with everything else in boxes. So the history is sketchy.
Thanks for everyone’s insights. And for helping me with the correct terminology.. 
Copy and paste to see video.
photos.app.goo.gl/E4xJDapvDzzxyTvB7