Undersink waste pipe leak

As hopefully shown in the attached diagram, my under kitchen sink wastepipes have a vertical nipple, (1 1/2" OD pvc) coming down into an old J trap, then the short end of the J trap goes into a compression el, then into a straight nipple. I don't have one of those nipples with the 90 degree bend and flange at the end. My leak is at the compression el.
My question is: I wanted to use a Fernco PFT 150 flexible J trap, which would catch the short end on the flange of the nipple and secure it with a hose clamp, but it does not look like the short end of the Fernco flexible J trap can just be used with a new compression el. I guess I could remove the straight nipple (the other end of which goes into a brass compression fitting) and replace it with one of those flanged nipples. I could just use an entire kit which would include a rigid J trap, some now have a flexible portion. Problem is, I hate to disturb that brass fitting because it doesn't look like it is leaking. If I disturb it to insert a new nipple with a flange on the other end it could start leaking. Thanks in advance for any advice.

Get a new trap and do it right. Flexible stuff is pure garbage!
If the other stuff starts leaking replace that too.

Get the whole thing new. Pay very close attention to how it is assembled. The plastic washers are tapered and must be install properly or they will not seal. As Redwood says, do not use the flexible pieces. Yeah, they look like they will really make the job easy, but they are a disaster waiting to happen. They sell these pieces of crap to unsuspecting homeowners who have little or no experience in replacing traps. Then when you come back when the fail, they can sell you something else. I prefer the metal traps. They may cost a tad more than PVC, but in my opinion, they are better and look nicer too. Since you have no experience with traps, it would be wise to go to a real plumbing supply. The folks there can make sure you have what you need.

REmove everything from the brass "compression ring" outward and replace it with a tubular brass or plastic trap. If it is done correctly it will NOT leak.