En-Tee700 (I like your Forum name, BTW), welcome to one of the best Forums around. We've got quite a few people who still have these old bikes and we've got quite a few who used to have them and have hung around because of friendships they've established. I've been on the Forum since back in '10 when Motorcycle Larry established it. He was an entrepreneur who made/marketed a pretty good range of accessories for the NT and a few other bikes. Larry died and his family wasn't up to maintaining the Forum while they were trying to save the business.
Our current administrator, Mellow, was the admin for the ST-Owners Forum and stepped up to take over this one as well. It's a good group with lots of experience and wisdom. Our humor level can be both pretty good and pretty bad, if you know what I mean.
Introduce yourself to the group -- share some of your motorcycle experience, etc.
I can't help you at all with advice about Delkevic or any other muffler options. But be assured that somebody will step up!