When I put a brand new 15 amp in the bike hasn’t died yet. I would assume the fuse may have been old and starting to degrade?
It is accurate to assert that these old motos' fuses
can degrade; furthermore, the sockets into which they fit can oxidize so
they should be inspected also. The fuses can be replaced by whichever fuse at the hardware or auto parts store has the correct blade width and amperage. They don't need to look exactly like the OEM version.
An important tool for Bricks is a can of DeoxIT
D5 electronic contact cleaner. All electrical connections should be cleaned and treated with DeoxIT. Some auto parts stores might have it but I get mine at a musical instrument dealer who has a large stock of electric instruments and electronic equipment. Of course, you can buy it online. You should make it your task to clean connections wherever you find them until you've cleaned all of them.
Good luck with your Brick.