Section 3 If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified. /section 4 The Congress may by law provide for the case of the death of any of the persons from whom the House of Representatives may choose a President whenever the right of choice shall have devolved upon them, and for the case of the death of any of the persons from whom the Senate may choose a Vice President whenever the right of choice shall have devolved upon them.
Also, the 12 Amendment provides for "contingency elections" whereby the House of representatives elects the President and the Senate the Vice President. As the two are now linked (they weren't at the time; imagine if Trump's veep was Hillary!) so that could make for an interesting Constitutional paradox. Pelosi might be acting President temporarily, but I rather doubt it, and doubt she'll have any legislative authority. But you can't put anything past these people.
This is going to be interesting, though. Every state where the Democrats control the office of Secretary of State will be in peril, as the SS must certify those states in time for the Electoral College vote. I suspect the Democrat-controlled states will simply refuse to certify their state elections.
Or they will certify them with serious vote irregularities (if they can make them go for Biden).