Help building crimson fist army. Discontinued minis

Hello. I used to play warhammer fantasy like 15 years ago. I want to try warhammer 40k now. I'm used to the rules since I play with some friend's army, but I want to build my own army of Crimson Fist marines.

The thing is, I'd like to use Pedro Kantor in my army, but his figure is discontinued. How can I use him then? Can I pick a random mini and say it's Pedro Kantor? Will that be allowed in an official tournament? There isn't even a section for crimson fist in the official games workshop store, only imperial fist, but they have their own rules.

Also, some tips on army comp/good minis and painting techniques/colours to buy for them is welcomed.

Thanks in advance.