What I've done on my scratch built electric conversions of glow type models, is before covering the model, temporarily install the radio system, run a dowel through the fuse sticking out through the nose of the model. Then put it on a balance jig.
Then place the ESC, motor and battery pack on that dowel, and move it forward or backward until it balances. Once that's established, then move the motor an inch or two forward to allow for the covering weight and so on. Now, just build up the fuselage nose to the required length. Crude, but it works.
Much better to have the nose slightly longer than needed, than to have to put lead shot in the nose to balance.
FYI, just about every glow conversion to electric I've done wound up tail heavy. And I use those overweight A123 cells.