Actually, the Monarch *controller* does not lower the field at higher speeds either.
That function is handled by the control pot, which has a special "double-half-element" construction to change to field weakening/constant armature voltage at about half rotation. The controller simply responds to this control input.
With suitable conversions due to differing pot resistance in monarch vs the usual controller, there is no reason why a couple of SCR controllers would not work USING ORIGINAL POT.
Several folks seem to be in various stages of doing this, and the people at KB Electronics seem to have some experience at doing it. Marty Escarcega is one of them, check posts below.
[This message has been edited by J Tiers (edited 08-29-2002).]