Hitler somehow takes out the Soviet Union in 1941 (a very tall order) and spends 1942 building up. But this works only if somehow the US is not the war.
Frankly I have to agree that there is no way a 1940 Sealion works.
The other possibility is Hitler decides not to attack the Soviet Union in 1941 and instead spends the year bombing England at night, while going with the Canaris strategy in the Med 1940-41. AND actually makes proper invasion plans. The superiority of the FW190 over the 1942 Spitfire Mks gives the Germans a better chance at air superiority, and the potentially the Germans can build more parachute units, transports and dedicated assault sea lift.
By that time the Germans could have developed proper anti-shipping doctrine for the Luftwaffe (after the historical Crete campaign that they actually did).
The summer of the 1942 sees the Allies very seriously stretched by Japan in the Pacific, the Battle of the Atlantic is at its height (and not going well for the Allies), and and US ground forces are pretty sparse (only a couple of divisions are in the UK at this point, most of the rest are training, hurriedly being deployed to defend Australia, or tied down elsewhere).
Still probably won't work, but the most likely possibility of any German success.
Unless Stalin invades from the East....
Hmm, this actually might work, counting that Stalin and Hitler be buddies (Stalin would not dare invade Germany if it was pushing back "evil Capitalist nations", and even maybe after that because when the invasion ends there will be no two-front war against bigger enemies, counting that Hitler kept quiet about Japan's DOW.
It seems that Sealion has no chance of succseeding before late 1941 or early 1942, even counting that the UK grows weaker. I think the only possibility of a succsess is to mobilise the entire Kriegsmarine and Luftwaffe, and have troops sent by both sea barges and via paratroopers, but bombers have to destroy the AA positions before anything could be done, since a lot of airborne transports would arrive to Britain. This is stretching luck and having the UK loose the Battle of Britain to Germany.