Ok, maybe I can help a little. The 3996X steel idler gear will work in a single speed setup and yes for a stock 3906 setup they need to be ground down some (really not a huge deal). I am using one right now in a single speed setup (albeit not the G1 but a "custom" job).
Another thought may be these aluminum ones on ebay
http://cgi.ebay.com/ws/eBayISAPI.dll...m=250329393785
As for the FLM half case, they were made and here is what I see in the store here:
CNC Machined E-Maxx Transmission Half-Case
This is the CNC machined E-Maxx transmission half-case from FastLane. It's made to house the G-Maxx single-speed conversion for the E-Maxx transmission.
Price: $40.00
49.10 CAD | 26.78 GBP | 30.75 EUR Out of Stock