Technically is possible to install "FES like" system on old gliders. Problems starts with papers. Gliders are usually certified by CAA (FAA) and this modification is practically impossible without change of Type Certificate. That can be done only by Type Certificate holder so it means very close cooperation with producer. Or you have to move your glider into experimental category.=thanks big L/D motorless glider is good flyer...
then,aspatially in case of old single place gliders it is possible to mount on the orginal nase motor/propeller (FES like!)
Also MTOM can be a problem for gliders without water balast. As you cannot make large structural modifications you have to install battery somewhere in fuselage behind pilot = battery holder must survive a lot of g-force (over 12g if I remember well). Also pylon must be designed the way that during impact cannot fall into cockpit.
You have to follow technical requirements also when you make new design, however than you can install batterie for example in the wings.