I have an idoc number in IDOC_INBOUND_ASYNCHRONOUS

One usually assigns a process code, which may have a custom (or an SAP-written) function module assigned to the process code for the message type. Research IDOC / EDI if this is what you're trying to achieve. If you're trying to reprocess an IDOC with another function module, you'll find the IDOC contents, by EDI number in EDIDC for the control record and EDID4 for the IDOC content, where the SDATA field content from EDID4 should be copied into an internal structure/table described to exactly match the DDIC structure cotained in the segment name field.