if you intend to replace the 20" wheels with 26" ones why do you think you still need a oversize chainring?
an alternate concept.. manufacture a chainring to fit the hubshell spoke hole circle of a SRAM Hybrid dual drive rear wheel hub.
laser cutting CNC machine will handle it, using stainless steel 1/8" to 3/32" thick stock.
fixed to the spoke holes , it will be the final drive to the rear wheel.
the hub itself functions like a jack shaft the crank turns the cassette on the right end of the hub,
inside the hub there is 3 speeds .. 1 overdrive 1 reduction
The overdrive in a bigger wheel resolves the need to change the stock crank.
such transmission hubs in a simpler 3 speed, are common now in Adult upright trikes ..
having the hubshell drive the rear wheel indirectly rather than in a wheel
Last edited by fietsbob; 09-12-14 at 08:44 AM.