Hi there,
I just wanted to mention that if you want to double the current
output then the first thing you would have to do is get a transformer
that puts out 8 amps instead of 4. Im not sure if you want to go
that far or not as this will be somewhat expensive unless you can
find a cheap transformer somewhere. The voltage needs to be
the same as the old one too. Of course increase the fuse
rating to 10 amps. You may also have to parallel the circuit board
traces with some copper wire to increase the current capacity
of those conductors too.
The rest would probably be easy, like double the SCR rating,
parallel the 0.25 ohm resistor with another one, double the
bridge rectifier rating, but this would need to be tested.