Yes you can still buy Brylcreem.
Black Hair dye is not a good idea as it can look Green when applied to some hair colours. Better to go for a dark brown wash in/wash out type of colourant. I would try with Brylcreem first as that has a darkening effect on hair anyway.
Forget the shooters ~ The Krays did not habitually carry guns and in this day and age, even going to a party, with your track record you are bound to attract the attention of the local police and are likely to be confronted by armed officers.
The Krays relied far more on a threatening manner ~ best achieved with narrowed eyes and a quiet voice.
Think Vinny Jones. His whole manner is one of restrained (just) violence but the only shooting he does outside of films is at clay pigeons.
Above all you need to be immaculately turned out as this was the Krays big hallmark.