yes - I did put in ARP head studs, ARP dont sell it for daihatsu CB80 but there is a ford part number that you can use. The car already has strut braces on the top of the front shock towers and also a tie brace under the engine room. its pretty nice, but please remember the car was not built with durability or super long life in mind either. It was basically a cheap and fast hatchback.
The block is not cross bolted and neither deep skirted, it will flex at high cylinder pressure. So when we had a subaru wrx turbo on it running a total of 5 fuel injectors in it with a larger throttle body. we had good flow in and out the head but then you get limited from the ECU timing and fuel control - because its very basic. Think of it as the engine is already quite highly stressed, you are squeezing more power from it. Something will either stop you or sacrifice itself.
the sacrifice was usually the small diameter clutch plate.