There are a number of solutions including ready made overflow boxes for undrilled systems. However, if you can find a way to drill the tank, that would be preferable. This does not have to be the bottom of the tank either, many overflows are designed with holes drilled near the top of the display tank. The issue with over the top style overflow systems is that they are prone to losing 'siphon' and this will starve your return pump if not noticed in due time. There are other consequences as well, hence the better way is to drill the display for proper drain.
As far as return pump size, this will depend on the size of your system and the amount of head pressure the pump has to overcome to return the water to the display.
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