I cannot afford to pay the mut'ah in one lump sum. Can it be paid in instalments?

Yes; mut'ah can be paid in instalments if your ex-wife agrees or if the court orders it. What matters is that you do not use inability as an excuse to delay someone's rights. The Prophet ﷺ said: “Delaying the payment of a debt by someone who is capable is an injustice” (Sahih al-Bukhari 2400). Put forward a realistic proposal, for example a monthly amount and a fixed payment date, based on your actual income. Record the agreement in court so that both parties are protected. Pay by bank transfer so there is proof, and do not mix mut'ah payments with child maintenance. Fulfilling rights honestly also makes your relationship easier as parents to your children. Read the explanation at rights after divorce.