The 6303 is a descendent of the 6303 so the question of 6803 support naturally comes up. As far as I can tell the only difference of relevance is that the 6303R has Vcc on pin 21 and a standby signal on pin 21, whilst the 6803 has Vcc on pin 7 and Vstandby (for the iram) on pin 21.

If I am reading the manuals right it should be sufficient to replace R4 with a wire link to run a 6803 processor. Once I have one to hand I will test.