How the little piggies would grunt, when they hear, how the old boar suffered.

How to say that quote in esperanto?

Kiel porkidetoj gruntus, kiam ili aŭdus, kiel maljuna virporko suferas.

Although 'would grunt' looks like a conditional (do gruntus) it actually has another meaning here. It describes a habitual action in the past.

Every day we would go for walks - Ĉiutage ni kutimis promeni or Ĉiutage ni amis promeni.

Underlying this use of would in English is the notion of 'volo'. and the realisation of that 'volo'.

Kiel amis la porkidetoj grunti kiam ili aŭdis pri la doloroj de la olda apro.

How the little piggies would grunt, when they hear, how the old boar suffered.

Shouldn't it be "when they heard"?

Kiel gruntadis la porkidetoj, kiam ili aŭdis pri la suferado de la maljuna apro.

If you are talking about the dying words of Ragnarr Loðbrók, then the "would grunt" surely doesn't refer to a habitual action in the past.

A bit of context: According to legend (?), when Ragnarr Loðbrók was dying at the hands of the English king, he told him: "How the little piggies will grunt, when they hear, how the old boar suffered" or "How the little piggies would grunt, when they heard, how the old boar suffered

Him being the "old boar" and his sons (or his whole people) being the "little piggies". This is a menace: When my sons find out how I died, you will hear their war cry.

So maybe in Esperanto: "Kiel la porketoj gruntos, kiam ili ekscios, kiel suferis la maljuna porkego."
The "kiel" used in the same sence as "kia" in "Kia ĝoja festo!" as found in the Ekzercaro of the Fundamento.

The question whether it is "will" or "would" is purely academic, too, because norse (as all Germanic languages) doesn't have neither a future tense nor a conditional mood. Also in current Germanic languages the future tense is generally derived from an expression meaning "to want to", "to intend to", "to become", while both conditional and subjunctive are covered by a mood called conjunctive. English is a bit of an oddball, because after its creolization under the French superstratum, even its verbal grammar became more romance than germanic.