When I converted mine, I just bought the a 2spd nitro Sav tranny and threw the same
Flux idler in there (cheaper that way at the time, $15 for tranny and ~$7 for the gear vs ~$35 for flux trans on ebay.) Also, if wanted, you could put thier new
HD idler gear in. Its 39T vs 44T for a diff range of gearing.
I went thru all the part #'s a while back, and its all the same as the 2spd, except for the idler. There are alot of parts to change if you were to start from the 3spd (also why I did what I did.)
Personally I have the Cen gears, 44T idler, 43T spur and currently a 16T pinion. 40 series mashers. ~17:1 IIRC. This is on a 1100kv 1521 on 8S. So far gearing seems about right.
I'm not building it for top speed. It may do mid/low 40s I think. Dunno as I haven't really tested it on a spd run. Plenty fast for this big monster IMO.