fleebell,
A sprague rated for 10 Ft./lbs. or more will do it with a good safety margin.
These engines produce somewhere around 2 H.P. out of the box...give or take a few tenths.
The basic formula is Torque = 5252 X H.P. divided by rpm
Torque at the crankshaft is about 1.5 Ft./lbs. @ 6000 rpm
There is a 4.1:1 internal reduction in the engine.
4.1 X 1.5 gives approx. 6 FT./lbs. @ the engine drive sprocket.
If you plan on another reduction prior to the sprague simply multiply 6 X the ratio of the reduction for a final torque value.
This isn't precise, as the actual H.P. and rpm where max torque is produced are unknown. It is close enough to choose the correct sprague, as you will want one that is over-rated anyway.
BTW...A commercial quality sprague, (One-way bearing), will outlast a bicycle freewheel 100:1.
Jim