Accessability issue: How to get usable plain-text section in my custom code email?

Hello, I am working with a long-time Constant Contact customer. They are sending out an email using a custom code template. There are complaints from their recipents 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 Custom Code only allows the HTML section to specify, I must either 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. Can you please tell me how I would accomplish having a plain text section with at least line breaks? Thank you. PS is there a way, from the Constant Contact account site, to preview what the plain text will look like?