I guess isolating Leningrad by capturing ports east of it is now out of the question?
Bombing port in Osinovets may be an alternative solution, but it will require many turns of bombing before it will cease to work (due to 2 bombings per turn limit and smaller impact on the port than in the past). I guess it would take time until Blizzard or even until New Year.
Maybe you could try to capture all hexes of the city south of the major river, including Leningrad proper, and then attack the hex currently in the center from 3 sides? You'll have to send your best infantry, full of pioneers, under best available leader to do it.
I don't remember this now, but the "level 2" hex east of the city is not a swamp hex (and is no longer a level 3 fort), so you'll get more CVs per counter if you'll attack it with panzers, not infantry. Get your best panzer leader to do it, give him 6 strongest panzer divisions, send them extra supplies (remember to put an airbase in nearby hex) to get their ammo over 100%, and give them as much support units as possible (with pioneers/flame tanks or highest CV that you can find). Before the attack, bomb the hex relentlessly with all available airpower. Perhaps launch some spoiling attacks first, but you need to be able to put some units across after the attack will succeed, so perhaps some units have to stay at the river line and don't move (or even don't participate in attack).