ATS has an L1 driver that uses FastDDE. You can download a demo and the manual at the mentioned website.
But that's about the only L1 driver I know of.
I think you won't be able to remain in this state for too long any more. The 105U is an obsolete PLC, as is the L1 protocol. Both have been declared end of life some years agoo. Nowadays Siemens has started the end-of-life cycle for all S5 PLC's. Within about 10 years you won' find any support left for these PLC's anymore. I would strongly advise to consider upgrading to a more up-to-date PLC (S7-200, S7-300 or even another brand). You will surely find easier ways to integrate SCADA with those newer PLC's and in the end it might even be cheaper to start right away with a new PLC.