Go for original CB100 engine, its the best option, forget about cultus engine as it will cost you around 40k for non efi and will require changes in mounts, while the original petrol engine will fit like a charm, plus you wont need gear box.
You can sell ur current engine at about 8-10 K and you can get very neat engine at max 25k (with gear) and without gear it will cost you around 20k .... you can get petrol engines from 15k to 25k, but 15k engines are Singapori engines, go for japanese as its better option.
So your overall cost for swap will be around 15K ...
if you go for cultus engine, you have to buy full engine (with gear box) and with would cost you over 40K (all setup even after selling ur current engine at 10K)
If you wants a 1.3 engine, then 4EFE is the best option, but thats very expensive now days, the whole setup will cost you around 55K
I think, the original 1.0 engine of daihatsu has enough power, it evens give very good pick on 88 dadu charade which is quite heavy than the 86 model. Its also fuel efficiant and gives around 16KMPL in city and around 20 out of city ... plus as its a carby ... the tunning is easy
rest its upto you ... go for 1.3 4efe if you have spare 50k ... otherwise 1.0 is best option.