HTML email templates can be a bit tricky now that you have to contend with responsive design as well as the variety of different platforms. Pulling HTML from one system and dropping into Marketo has never worked for me. Invest in Litmus and test test test how the emails render across mobile and desktop.
Most importantly don't forget to migrate the unsubscribe list.
Check to see that the records that you want in hubspot have successfully been migrated over to Marketo. Not all records from hubspot auto sync to SFDC.