According to the help file, export to MIDI was implemented in MPT 1.10 (June 2000).
And yes, do upgrade to the latest version, it still runs on ancient hardware and I myself have a copy of OpenMPT 1.25 on a Win98 machine.

OpenMPT is not all that different from what MPT was like back then.
Old ModPlug versions are known to write out malformed IT and XM files in various ways which will make e.g. resaving the files difficult in Impulse Tracker - which would have been the IT tracker of choice in 1998.