The materials used to construct the express scooter are cheap. which word could be substituted for “cheap,” if the speaker wanted to express a positive viewpoint? economical inferior second-rate substandard

Economical. This has the most positive connotation out of all of the options listed here. 
"Inferior" implies that the quality is lesser than other potential materials that could have been used.
"Second-rate" and "Sub-standard" also imply the same message. 
"Economical" has the most positive connotation, because it implies that the materials are monetarily cheap but does not really imply anything about the QUALITY of the cheap material.