If I were in your position, I'd be looking towards a cheap synthetic bag LONG before spending $100 in down and putting it in a sub-optimal and compromised DIY shell.
I have advocated several times on this site for the $100 EMS Solstice 20. It's an excellent bag, compresses well, doesn't weigh much, and is consistently warm. You can often find it for as low as $60, with some patience.
Now, that being said, if you want to get the best help on this project, take a few pictures of the fabric and the areas you want to re-sew. See if you can find out exactly what the original fabric is through the internet, contacting the company, or the tags. Once you have that info, bring this thread to the MYOG forum, where the experts can advise you how to make the best of this project.