If uou are really strapped andf have plenty of ROM, you can do it with 10 lookup tables.

clr cy
ones = lookuph1(high byte)+
lookupl1(low byte)
if ones >10 set cy and ones = ones-10

tens = lookuph10(high byte)+ cy +
lookupl10(low byte)
if tens >10 set cy and tens = tens-10

...

Have fun,

Erik