The TX750 swingarm swap is a very worthwhile mod in my opinion. As far as swingarm swaps go, it's about the easiest one out there. It's a virtual drop-in replacement. The 750 swingarm is exactly the same size as the 650 one with one important difference - the tubes are nearly 1/4" bigger in diameter. Practically everything else on the two arms is the same - shock mount location, pivot bolt, pivot sleeve, bushings, axle size and chain adjusters. The chain guard mount does differ though. You can mod the 750 arm to accept a 650 guard or just use a 750 guard.
These really aren't that hard to find. I bought 2 off of eBay last year. One cost me $29, the other $43. The $29 one was pretty rusty and needed a total refurb. I passed that one on to my buddy. The $43 one was pretty much pristine. Both the bushings and the pivot tube were still in excellent shape. I was able to install it "as is" without replacing any of the pivot parts. Measuring it, I found the pivot sleeve to be about .008" longer than the swingarm w/bushings. It took about 45 ft/lbs of torque to "set" it properly so the arm gently fell under it's own weight.
Now some install details - even though I don't need it at the moment, I decided to install the extra grease fitting in the center of the arm. The arm was all apart and I may need it some day if I eventually do install bronze bushings. The drilling and tapping operation fills the arm tube with metal chips. They can be difficult to clean out because it's all greasy in there. I avoid the issue by inserting cardboard tubing into the arm first to catch all the chips. I just use the cardboard tube from a roll of paper towels slit and cut down to fit .....
In the center, where I'm drilling and tapping, I notch the cardboard tubing out so the chips will fall into it .....
I also mod the pivot tube by adding an additional pair of holes in the center .....
This allows grease to come out there and fill the center section of the arm .....
Pump enough in there and eventually this would lube bronze bushings from their rear. But I still like to have the added external fitting too.
One minor issue you may encounter with the 750 arm install is clearance for the brake rod and pedal pivot. The 650 arm is dimpled in this area .....
The 750 arm doesn't have this dimple so clearance is tight. You'll need to flip the anchor pin around so it's thin head is on the inside, cotter pin on the outside. I also took the opportunity to swap in an earlier grease fitting equipped pedal pivot .....
Difficult to see and photograph, I do have a small amount of clearance. It gets better when you sit on the bike and the rear shocks compress some. If need be, you could bend the bracket on the pedal pivot a little to provide more clearance. I didn't need to do that on either of my installs. Both are a tight fit, but do have a little clearance .....
Last item would be swapping the old style Japanese grease fittings on the 750 pivot bolt for newer style ones. Our American grease guns won't fit these old style larger headed fittings .....