hi;
if i understand your problem correctly then you want to set a bit whose location is to be passed to you via r1.by using a lookup you can convert it into required code as follows:

set_p1_bit:
;p1 bit number from 90h to 97h
mov a,r1
clr c
subb a,#090h
mov dptr,#look_up
movc a,@a+dptr
mov p1,a
ret
look_up:
db 01h,02h,04h,08h,10h,20h,40h,80h

Mind you this destroys all previous contents in P1.you may need to modify the code as per your specific requirements.
Pranav