Accessibilty issue: how to get a more usable content-type: text/plain section in generated emails?

Hello, I am working with a long-time Constant Contact customer. They are sending out an email using the api (/emails endpoint with format-type 5, custom code). 

There are complaints from their recipients who need to read the plain-text version of the email, which is automatically generated by Constant Contact into the Content-Type: text/plain section. The plain text is one massive line with no line breaks. Unreadable and unusable for those who need to use plain text.

Because the API only has a place to send HTML section, it seems I cannot control the plain text section, and Constant Contact generates that. 

Can you please tell me how I would accomplish having a plain text section with at least line breaks?  I believe CRLF is recommended.

Seems that either there would be an api way to  a) specify the plain text section separately or b) set up the html that will cause Constant Contact to generate the plain text section with line breaks. I haven't found any info on how to do either of these things.