Sounds like your clutch is slipping. If adjusting cable does not help much then your clutch plates are fried. Sorry dude, but time to replace. Better put stiffer springs in while you're at it.
On the bright side, clutch work is like ten times cheaper than a transmission.
Tightening the clutch actually means loosening the cable which means more slack in the lever. Tightening the adjustment nut at the hand lever is what's needed.
I believe Nev's lockup kit is closer to $1200 USD but exchange rates change a lot. Unless you're making more than 150 HP you probably don't need a lockup. It works real dandy in drag applications though.
You need to make sure when the clutch lever is not pulled, the cable needs to be loose with no tension on it at the elbow lever on the engine case. If it is tight, that means the cable is holding the clutch apart and so the transmission is not getting engaged fully when the lever is let go.
Most likely, you ate away alot of material from the friction discs so now there's more space between the discs and you have to adjust the cable to get things engaging again. You should still replace clutch plates even if adjusting the cable gets her working. Your plates may still be glazed and they won't last much longer if you have to make massive adjustments to the cable.