If a TTR90 is too small, a GasGas 50 will be way too small.
The suggestion previously made was to fit a Honda centrifugal-clutch motor to a Honda TL125 trials bike. The TL125 is a full-sized frame, and the centrifugal clutch motor is very docile, and will fit with a minimum of work.
A TY175 in standard trim is incredibly docile, but has a manual clutch.