Legacy formats and software is always a crapshoot, I've found a few leads that might help you.

From an article back in the 2000s "A plug-in for 3D-Studio MAX that allows importing and exporting SPX data into and out of 3D Studio" Maybe try and hunting that down?

This legacy software might work for you

Or some of these might.

It looks the the German company that originally developed it changed from Interzart to Scanbull Software around 2006 but I can't seem to find any other information about them.

Their software was 3D Editor Marlin, so unless you get extremely lucky on ebay you might be out of luck.

This git repository of old software has several versions of Dimension 3D-Systems GmbH - Open SPX Plugin which might help.

This was all the information I could find with limited research time, hope it helps!

Legacy formats and software is always a crapshoot, I've found a few leads that might help you.

From an article back in the 2000s "A plug-in for 3D-Studio MAX that allows importing and exporting SPX data into and out of 3D Studio" Maybe try and hunting that down?

This legacy software might work for you

Or some of these might.

It looks the the German company that originally developed it changed from Interzart to Scanbull Software around 2006 but I can't seem to find any other information about them.

Their software was 3D Editor Marlin, so unless you get extremely lucky on ebay you might be out of luck.

This git repository of old software has several versions of Dimension 3D-Systems GmbH - Open SPX Plugin which might help.

This was all the information I could find with limited research time, hope it helps!

UPDATE

I might have found a legacy webplugin that could be of use! See if this helps!