Hi Bugdevheli,
Could you not get away from the belt drive! I am aware that belts will save weight , but they are a scource of possible problems, just of late I have been storing some textile machinery from a Swiss manufacturer, as any machinery interests me I was interested to see some very small 90Deg power transfer boxes that were coping with something in the region of 100 SHP these transfer boxes were made from what seemed like Ali or magnesium with power shafts about the same dia as the tail rotor shaft on the R22, just thinking on a little, a transfer box fastened directly onto the side of the engine would present less mounting problems further down(or up) the transmission shaft to the main rotor .
Anyway keep telling us how far you have got with your Mk3
Regards
PeterR-B