Using the adverb "now" with a tag question [duplicate]

As you say, the correct form is

People can hardly motivate themselves now, can they?

The other sentence with can't in the question tag is not correct. Where the question tag indicates that the speaker is expecting the hearer to agree, the tag verb negates the main verb.

People can't motivate themselves now, can they?

This indicates an opinion on the part of the speaker, and anticipates agreement.

The reason that the correct tag for the first sentence appears to repeat the main verb is the use of hardly. Hardly is a Negative Polarity Indicator — almost as strong as not — and therefore the tag verb has to be positive.

To use can't as the tag verb, the first statement has to be positive: "People can really motivate themselves now, can't they?" Again, this assumes that the speaker expects to be agreed with.