The higher amperage rating is not really relevant - that just indicates the max current that the power supply can provide. If the device pulls less than that, it isn't a problem.
However, the polarity of the power supply is very relevant. If it's is the opposite of what the ZX81 needs, and you powered it up, then yes, you may have damaged something.
The polarity should be shown on the power supply - usually it's a diagram with three circles which illustrate which part of the connector is positive and which part is negative. There is often times one on the piece of equipment as well. Those diagrams need to match.