Replacing flat drive belts alternatives?

[Craig Ruff via cctech]

I would like to have both the drives in my HP 9895A working. Since flat belts 
for old equipment are becoming nearly impossible to find, has anyone had 
success replacing a flat belt with a toothed?  Either by reversing the belt or 
by also replacing the pulleys?  Or by the installation of an idler pully to 
correct for belt stretch?