we have a lead A with id 114427000000381xxx,

After it was converted, we can find the contact/account with the link

https://crm.zoho.com/crm/EntityInfo.do?module=Leads&id=114427000000381xxx

But how to find the contact/account id with the converted lead id with api?

We get the following result with https://crm.zoho.com/crm/private/xml/Leads/getRecordById?authtoken=xxx&scope=crmapi&id= 114427000000381xxx