There's actually a couple different ways you can do that.
1. Machine a bushing that the gear rides on to offset the idler gear on the big end of the idler pin, or
2. Offset the entire idler pin.
If you choose #1, you need to either press fit the bushing onto the pin, or Locktite it. There's a special flavor of Locktite that's made for locking gears onto shafts. Can't recall what it's called offhand.
If you choose #2, no need to Locktite the bushing in place. The shoulder on the pin will bear against the crankcase wall when you tighten the nut.
I turned down the idler pin where it passes through the crankcase wall to accommodate the ID of the eccentric bushings. Then I machined a series of brass bushings 0.010" to 0.050" offset. Held them in a 4-jaw chuck clamped to teh table of the mill. If you don't have a mill you can also make it entirely on a lathe if you don't mind messing up your headstock/tailstock alignment.
Tried each bushing in turn until the backlash just about disappeared. Once it was in place and I had the eccentric pointed in the right direction.
http://www.microcogen.info/index.php?topic=936.msg26850#msg26850The difference the offset idler bushing makes in engine sound is amazing. No more "clack-clack" as you crank it over. Very little backlash. Engine runs with much less mechanical noise.
I suspect the problem with broken gears on these engines is 90% attributable to excessive backlash caused by improperly located idler gears.
Quinn