My question is on your prints you have a 1.5" square tubing sliding over 1.5" square tubing.....? oh yeah & do you think your height will work for a 99 YZ 125?
Check the drawing again and you'll see that the lift uses 1.5" i.d. and the stand 1.5 o.d. so they should slide over each other no problem. If you're concerned just test fit the pieces at the store. You can go with a looser fit than that no problem just don’t go so loose that the stand/bike rocks back and forth.
Not to worry too much about friction or ease of lifting with the design it works well. A longer lifting arm could be used but like Dave said be sure to reangle it as well or you'll end up hitting the floor before you get the lift high enough to lock in place. Even though the lever is on an awkward angle during the initial part of the lift is quite easy because you're only taking the weight off the suspension at first. The same when the bike is lowered. People have asked if it wont come crashing down, no the bikes got suspension.
Don't hesitate to place a washer on each side of the pivot points. It will make it easier to lift as David mentioned. That's a detail I forgot to add but is something I’ve done.
Ya it'll lift an MX bike of the ground no problem. To get the maximum amount of lift measure the clearance under the frame and adjust the height of the lift diagram to meet your needs. This design will lift a KX250 off the ground so the YZ125 shouldn’t be a problem but its worth double checking and lengthening the lift ( page 3 of 3) to maximize your lifting height.
You don’t have to follow the plans exactly, you can change the size of the materials no problem. Don’t hesitate to use 2” o.d./i.d. material and 3/4inch square material for the leg or what have you. The important part is the location and length of the link.
If you need a total height let me know and I'll check my YZ250 .