Just to make sure we're talking apples and apples, you've looked at the Grubee site to confirm you have a GT5R (Super Rat), so we're not talking apples and oranges.
According to the Grubee Website, the Super Rat has a 10mm nut as the older style engines have a 8mm nut. So my guess is that the end of the shafts are different diameters. Maybe a possibility to tap an older style magnetic rotor to 10mm and switch out to an older style coil and CDI. You'd would have to drill and tap some mounting holes for the coil. I don't know how thick the engine case is.
I have a post in for someone to measure the diameter and thickness of the older style magnetic rotor for my own project. You could compare the measurements of the older style magnetic rotor to the diameter and thickness of your Super Rat to see if the older style would work with your Super Rat.
Before you do anything, I would check with some more experienced forum members on my suggestions.
Good Luck,
Chris
AKA: BigBlue