Woops double post. Newby here.

Is there a suitable Daihatsu piston around 78mm that has drilled instead of slotted oil returns?

Its has been done before, however the metal left between the piston top and the 1st ring land becomes weaker, and typically fail. Ideally try and find a piston where the top ring land is a bit lower down, or buy A15 pistons. They can be had cheap enough. If you are wanting a flat top type piston in an A15, there are several options available. There are a few articles and thread on what others have used in their A15's. Most involve enlarging the con rod little end to 18mm ~ 19mm piston pins though. The rods are up to it.

Has anyone here used A-14 pistons in an A-15 by machinating to tops to lower the piston deck height? what were the results? If it was done, what pistons were used?
Thanks,
john