Have a Pitt Bull Motorcycle lift that came with an Omega 6 Ton 10065B bottle jack . The bottle Jack is in need of a rebuild or replacement as it doesn't lift 100%, even after doing the tips n tricks online of how to bleed it it's clearly time for new seals or new bottle.

This is the current Bottle Link to Jack

Found a rebuild kit for $15, which I have received, the main reason it needs to be rebuilt is the release valve has a welded on extension making it safer and easier to release and lower the lift. Also the lift bar has a recess for the top of the lift to fit into, so that needs to be a specific size and it's drilled to bolt to the jack base.

Prices online run from $40 to $100 plus shipping for a new Omega, local Hydraulic company wants $100 (hourly rate) to install the kit.

Question is, how difficult is it to install the kit, or, is it worth just getting a new one with the $100 vs rebuilding the old jack.

Haven't utoobed the howtadoit video yet...

The jack is on a PitBull Motorcycle lift that I got from a friend when he moved out of state, at one time they were the top of the line in lifts, over $300, USA made, other than the bottle jack.

I bought an 8 ton bottle from Harbor Freight but the cap on the lift rod is too large for the receptacle for it on the lift and the only other HF option is a 4 ton, question being if a 4 ton is a good substitute for the 6 ton, considering the max weight being lifted is just under 1000 pounds..

Chinese bottle jacks are pretty cheap and a rebuild isn't likely to be cost effective or successful.
I'd just take some measurements and buy another. An American made would be preferable but likely much more expensive. Don't worry about the brand, the all appear to be made in the same place and same or similar (low) quality.
There's a reason they use a Chinese jack on an otherwise American tool. The US made versions cost about 10 times the price.

If I was shopping for a US version I would check to make sure there is a rebuilding kit available and buy it at the same time.

You already have the kit & bottle jacks aren't complex machines. My guess is you'll cuss more cleaning up the oil than doing the rebuild.

From that pictore, it looks like 6 bolts, and a $10 jack to fix the lift.
Id replace the bottle jack and call it a day.

I would feel ok using a 4 ton jack to lift a 1000lb motorcycle.

There is leverage involved in this jack's use. It is not a direct lift of 1000 lbs. The jack is doing more work than a direct lift, this is apparent in that the motorcycle goes up at a faster rate than the jack.
I would rebuild the jack, if it fails get another 6 ton replacement.