Hi. I have a 2016 Trek Superfly 9.6 (Hardtail). As I'm getting older, I would like to have a little assist going up hill. I thought getting a new electric mountain bike but can't since the cost of new one is out of reach. Is there a kit that I buy to convert my existing bike? With disc brakes, I'm guessing I'm limited to a mid drive? If so, any recommendations? Thanks in advance.

You're not limited to a mid-drive, but IMO they're best for off road. Look at a BBS02 from Luna Cycle or others. The total cost will be < $1,000 with a good battery.

Thanks for the suggestion. Looks like Luna Cycle is out of stock but I was able to find it on Amazon. I read somewhere that this is not recommended for carbon frames. Is there any truth to that? Thanks!

I haven't seen anything to support that, but since you "jam" the bike against the downtube, it would be necessary to cushion the frame IMO (I've used a rubber strip on all my installations). Additionally since your bike likely has a 73mm bottom bracket, you won't be able to use the "jam" nut for your installation and need to use Locktite on the center nut securing the unit. You might go to endless sphere and determine what others have done since I've only used the BBS02 on steel or aluminum hardtails.