Is what the questioner's father is suffering from, namely heart disease and the implantation of a pacemaker, a punishment from God because he took out an interest-based loan for a failed project, knowing its unlawfulness, even though interest-based loans for projects are legalized in their country?

It is not permissible to borrow with interest (riba) to finance projects, for usury is forbidden and is only permitted in cases of dire necessity that would lead to destruction or something close to it. Increasing income and developing trade do not fall under this category. Your father's remorse is well-placed, for remorse is repentance. He must seek God's forgiveness, resolve not to return to such an act, and increase good deeds. For whoever repents, Allah accepts his repentance. Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" Illness does not necessarily result from unlawful acts, but disobedience has ill effects, and Allah may hold a servant accountable so that he repents. So let him praise Allah for the repentance and be steadfast.