I glanced at the datasheet. It appears to have a Timer 2. In normal 8052 T2 can be used as baud rate generator. It uses a different divisor off the system clock. t0/T1 use a divide by 12 and 32, T2 uses a divide by 12 alone. So with a reload count of 29, you should get 31779, which is a lousy 8.8% difference.

You could use a switch to switch bewteen 2 clocks. Just be sure to be in reset while you do this.