I have completely stripped out a 10 gallon and resealed it! But if it is old, I agree, it doesn't have to be pretty. A very very generous "bead" and make sure that you spread the new silicone out past the edge of the old. I watched a youtube video of how to use masting tape to create at least a 1/2" band of silicone on all seams with neat edges... so using the same theory, make your "bead" very big and make sure the new silicone spreads out past the old. Find and watch the video if you can for a sense of what is needed.
Spend the money on a second bucket if you need to, keep the filter, heater, fish in a bucket. If the filter won't fit use the heater and an air stone. Do extra water changes. Old tank water isn't as important as keeping the gravel and media wet and not allowing it to dry out. Silicone takes a day or two to dry I think.
When you refill the tank, take you time to fill it completely empty. Take several minutes to raise the water level to full so that you can see if another leak springs up. I raised the water level a gallon at a time and waited 10 minutes, added another. You might find that all is good at a height of 15 gallons and then it starts to leak again. So you then know to redo the silicone at about 4-5 inches from the top and not the whole seam again. Once you know it will hold water, empty some of it and add your gravel, rock, plants decor. Don't do the work twice. Make sure it holds water first or you have to start all over again.
(Hey!! If I can do it!!?? Anyone can!!)