I don't hang out here but have some info. I have an old HP Velotechnik Streetmachine and the idler wheel failed. The newer models have a larger diameter idler. The shops I called didn't know what to order as a replacement.
Anyway, I ordered something that seems like the right size and I'll see if it works.
In the meantime, I experimented with no idler wheel. The bike seems to ride well without it. I do get more bouncing with steep hills or very hard acceleration but that is it. I kept the bolt in holding the tube retaining spring so there is no chain rub on the frame. I'm thinking of just putting a long chain tube in there and
leaving the idler off. I do have a stiff spring on the rear shock. I've only ridden without the idler wheel for about 20 miles so I'll report back if a problem develops.
Google on Power On Cycling, a Florida bike shop that caters to recumbent riders or on Hostel Shoppe, a Wisconsin recumbent shop. Either shop should be able to provide you with a replacement and both have online catalogs. You can also go the $$$ route and get a Terracycle idler. They make high-end replacemen idlers for most recumbents. Haven't tried one as I am terminally cheap and found the hard rubber ones from Burley that Power On Cycling sold in the past to be long-lived and work well for me.