Are you sure we are talking about KT - 4078 (I cannot find any picture with it online) or are you reffering to KT-4178 ? EDIT: It is likely the 4078 is the version with radio and 4178 is a version without radio. However, the mechanism is identical and they share the same belt if a 4078 does exist. I think this is the belt you seek: https://www.ebay.com/itm/324868190454
Thanks for sharing the pictures, I have never seen this one before. Please add some pictures from the inside to this thread when you do change the belt, so we can have a reference of its dimension. You can also measure its diameter.
Here is mechanism. Pulley far left is moving pulley. it accepts smaller diameter belts . Mine is 68mm . But i will get something like 70,73... It is 0.8mm thick