Issue with Customer Contract customization on upgrade from 2020 R2 to 2022 R1

Trying to convert the code below for the 2022 R1 release.  The PX.Objects.CT.CTBillEngine.ARContractInvoiceEntry graph has been removed, but I cannot find what it should be replaced with.

The code is updating some custom fields on the Invoice generated from the Run Contract Billing action.

    public delegate void BillDelegate(Nullable<Int32> contractID, Nullable<DateTime> date);
    [PXOverride]
    public void Bill(Nullable<Int32> contractID, Nullable<DateTime> date, BillDelegate baseMethod)
    {
        PXGraph.InstanceCreated.AddHandler<PX.Objects.CT.CTBillEngine.ARContractInvoiceEntry>((graph) =>          
        {
            graph.RowInserted.AddHandler<ARInvoice>((sender, e) =>
            {
                ARInvoice inv = (ARInvoice)e.Row;
                
                 // pull value from custom UsrCustPO field
                 Contract ContractRow = Contract.PK.Find(Base, contractID);

                 // extension name:  ContractExt
                 ContractExt ext = ContractRow.GetExtension<ContractExt>();