A complete explanation and schematic for this amp can be found at: http://www.electrosmash.com/marshall-mg10
A picture in that page shows an transformer with a secondary of 12.6+12.6 volts
Also in the data sheet of the output chip used in this amp ( http://www.st.com/content/ccc/resource/ ... 000129.pdf
) the manufacturer details the behaviour of an 50 watts transformer suggested for this IC
That power looks to be in accordance with the size of the original tranny appeared in the picture.
I guess that the fuse to place in a 220 volt environment to feed a 50 watt transformer will be roughly of 1 Amp
Good luck with your project !!