I went for 1000cc kit '77 S heads with a few mods, 9.5:1 pistons and Del Orto carbs in one hit.
Dual plugs came a few years along with valve seats and a mod to the squish band to adress pinging with the intoduction of unleaded fuel.
I have about 350,000 km on the bores and pistons with one ring replacement a couple sets of valve guides plus a set of guides when I did the valve seats.
Bottom end is still in good shape as are the cam and rockers.
Gear box has been trouble free.
I seem to have stopped wearing out universal joints.
I seem to be hard on the final drive but it does cop a bigger pounding since I went to USD forks 5 or 6 years ago.
I put 100,000km on the stock motor.
I did gearbox & final drive bearings as well as valve guides in the 100,000 as well as a couple of drive shafts.
With the stock motor I was always ringing it neck and got poor fuel ecomomy.
With the 1000kit I can get the response and performance I want with a turn of the wrist.
Motor will run on 91, E10 or Opal.
It runs fastest on 95.
I use 98 makes me feel better about running hot slugging it through the trees.
I average around 15- 17km /lt and have achieved 22-24km/lt running good dirt road in top gear at 80kph.
Stock motor would deliver in the 15-17 km/lt range never seeing better but sometimes returning 12.5 with the lowest ever 10.3 on a fang thru the hills.
I'm due for a new set of barrels (have been for the last 35,000ks)
I am still interested in a 1070 kit mainly for the mid range.
When I think back to what a blast this bike was when the 1000 motor was fresh and what it does now I'm thinking a fresh set of barrels, pistons & rings with a valve grind and new guides would give me back everything it had that impressed me as a 28 year old rev head.
If I went 1070 I'd need more brake and suspension upgrades as well as a higher 5th gear and posibly exhaust replacement with 38 or 40mm to get the best out of it.
The cost of the kit and associated upgrades is a bit scary .
For the same money I could have a very nice second hand enduro, a husky TE610 or a new SX85 for the budding MX rider in the family so I'll probably just go for another 1000 kit.
The Seibenrock power up kit with the standard heads and 32mm bings is reported to be a torque monster.
My bike is slow but the earth is patient.