?? are you saying you removed the electric motor and installed a belt drive tail? if so and your tail wont hold then theres a few things to check. first off what type of radio??  second did you try reversing the gyro? 3rd is the servo controlling the tail pitch slider moving the right way? 4th is your gain set right? 5th did you turn the belt at the tail clockwise  looking from the rear?
theres alot of variables there . by any chance did you search for forums on how to install the upgrade? im pretty sure if you did the search your  questions would be answered  google is your best freind 9 times out of 10 !  btw the 37 that looked probably were curious or couldnt answer your question! now if i come off as a dick then so be it but for what its worth  your better off scrapping that heli and getting a heli that has  a belt or torque tube or  replacing the burned up motor imho because it doesnt soud like you know what your doing. also one more thing how do you know the tail motor is bad? it could be a broken wire or a bad 3 in 1!  hope this helped a bit good luck.