As far as we know, it's not possible to miniaturize fusion. The mass of the shielding alone for a hypothetical miniature DT fusion reactor would prevent it from being used in anything smaller than a large ship. 'Advanced fuels' with reduced neutron radiation are orders of magnitude below DT in power density for given reactor performance levels, so miniaturizing them is even more in the realm of science fiction.
Solar+wind alone is also quite unlikely to stop climate change in that time frame and enough nuclear capacity will take decades to build. By all means keep converting to to cleaner energy but also build up the sea defenses and improve both building construction (better HVAC to deal with higher average temparature) and city infrastructure (better storm sewers to deal with predicted stronger storms and better electricity grid to deal with higher peak loads from HVAC and electric vehicle charging).
Including transit, shipping, airlines, and other industries where batteries won't quite cut it (not to mention deep-space exploration).