In terms of Rule 44(3) of the CGST Rules, 2017 if the invoices relating to inputs in stock are not available, the prevailing market price of goods on the effective date of occurrence of the events i.e. change of the scheme from Composition to Regular scheme, supplies becoming taxable which were earlier exempt and cancellation of registration, should be considered for estimation.
(ICAI FAQ PUBLICATIONS 06-09-2017 INPUT TAX CREDIT: FAQ NO. 62)