First time vintage bike owner

Potato959
Your engine is pretty much the same as mine. The clutch problem is likely something simple.
Take off the left side engine cover and check is the cable is seated correctly and the mechanism working as it should. You may have to remove the mechanism, clean, grease it and put it back. It has a ramp type mechanism.
See first pic

While I have never opened the clutch side I dont believe the ball which the clutch pushrod pushes against can fall out on the rod side. Its to big

See second pic

Hey thanks for the pics! That little piece moves with the clutch push rod. Seems to be operating fine. I'll clean it and relube it as suggested. Cable is seated properly as well. I'd like to also state that I rode the bike around town prior to changing the clutch cable. It was engaging and disengaging fine.

Clutch adjustment. you may need to turn the clutch release adjusting screw counter-clockwise first until hard to turn, then clockwise 1/4 turn. I believe the procedure is in one of the supplement chapters of the Kawasaki 750-4 manual. The clutch release mounting bolts must be properly tightened.

Manual: good version. 750turbo.com --- Downloads. 1980-84 750-4 & KZ700. 1983-85 ZX750.
yellow/gold cover w color wiring diagrams. get the one listed as >> GPz750 Workshop Manual.pdf >> 128 MB >> good scans.
There also had been one that says full english, but it's a cruddy, photo-copied version. all files except the last one should be pdfs (not exe).

Wiring diagrams. mbsween. bike-night. KZ750L3. Scroll down & click "The Bible" link.
the site has wiring diagrams & a 750-4 pdf manual but it's the same poorly copied version as above.
note: some of the wiring diagrams are mislabeled in the links: the 1981 Ltd is an H2, the 82 is an H3, & the 83 is an H4.
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Yeah I've seen that but it didn't work. Appreciate the links though!

Did anyone remove the clutch? There's a steel ball that can go missing.

Make sure the clutch release mechanism is cleaned, lubricated & operates normally. and is oriented correctly & properly installed w bolts securing it to the engine sprocket cover. make sure the clutch cable is properly routed & seated into the engine sprocket cover and release mechanism. one little goof & it won't work. Be sure to install a small cotter pin so that the cable end remains seated. When reinstalling the engine sprocket cover, I like to maintain tension on the clutch cable so that the cable end remains seated. Also, I place the engine sprocket cover on shipping boxes beside the motorcycle so that the clutch cable doesn't get kinked or anything. and so the cover doesn't get scratched.

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No one removed the clutch.
To refresh, I rode the bike home from the previous owner's residence. The clutch lever was INCREDIBLY tough to pull. I found that the old cable was kinked. I have not tried touring the cable back to its original positioning. I didn't think it would have that much of an impact. I will also assure that the cotter pin is secured. Will update when the weekend comes around.

I took a closer look and everything seems in place. The little ball is there, but the clutch plates were a little stuck together. Right now the veridct is cold weather and really thick oil. I'm going to clean the plates and put whatever oil the manual suggests.