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The statutory declaration is evidence of the agreement you had with your partner regarding repayment, and therefore could ground a claim before the court that he repay the debt on a more urgent basis. If you can also prove that he incurred the debt and that he indeed earns the income you describe then likely you could obtain a favourable order that he repay the loan at much faster rate, but how fast will depend on a fuller understanding of your ex's financial situation and obligations.
If you do have proof of his earning capacity then it may well be that if seriously confronted with the prospect of legal action, that he would simply deal with the matter, so if you engage a lawyer to press him for payment, it would seem sensible for him to simply deal with the matter or come to a more satisfactory repayment arrangement with you to avoid legal action. So it may be that legal help may assist without the need for actual litigation.
I trust the above assists.
Good luck and please rate my answer.