Monofilament line

Depends on what type of fishing you to as well.  If you troll, there can be a big difference between mono and braid.  Braid will rip your boards through the water, whereas mono will give you more controlled movement due to the stretch of the line.  I prefer mono when trolling due to this.  We've done tests with both and mono did better most of the time for us. We used to use magnathin for trolling as well, but since then I switched over to regular mono and there has not been a difference.  Baits may run slightly different but you can figure that out.

I,too,was a fan of Magna-thin. I liked it's thinness for for vertical presentations.It's thinness and durability worked well with trolling applications. It's limpness was great for casting applications.Now,for vertical,I use a braided line.For all others I use Gamma Copy-flex.It's limp,extremely durable,strong and long-lasting.It does not have the same lb.#/dia. ratio as Magna-thin.It's expensive but worth the money.Another co-polymer line that does match magna-thins lbs#/dia ratio is Sunline.I have no experience their co-poly mono but get some to try.It's much cheaper then Gamma

Power pro slick and tie on whatever fluoro you prefer.

Not gonna be the answer you want, but sometimes things just force you to move on and adapt.   There really isn't a line similar to magnathin, which I used extensively back in the day....aside from its weak knot strength, it was a wonderful mono, but mono is becoming a dinosaur for a reason, and the people holding onto the past and clinging to mono are just resisting change.    Several years ago, due to the way new rods are being manufactured (micro and semi-micro guides), I was forced to stop using straight fluoro, despite my resistance, and go to braid to fluoro leader....IMO, this is the route you should move to.     You will adapt and in time and be able to see that this set up will be an improvement in every way.   JMO, as like I said, there is no magnathin alternative.