Have a quick question regarding these types of abilities, e.g. Thorn's ability that allows him to exhert to recast any number of his combat tokens.

Does this apply to both spent and unspent tokens?

E.g. is he allowed to deal damage with his tokens, exhert and recast them and use them again in the same round?

The ability's description doesn't specify.

Concentrate on what the rules allow, not on what they do not specifically forbid.

So no, it does not apply to spent tokens.

In such cases as it does apply to spent tokens, this is, from what I have seen, specified in the text.

Mnemosyne is correct, if an ability refers to spent token, then it's spelled out clearly in the wording used. Otherwise the ability refers to the unspent tokens. You may cross-check our answers in the Reference Guide, pag. 5, Combat Actions:

Abilities that affect combat tokens only affect tokens still in a combat pool unless otherwise specified.
Example: The Dread Prophet’s Prophesy ability that recasts a token can only be used on a token in the hero’s combat pool, because it does not specify spent tokens.