Rework on Unity 3D game

I have source code
I would like to add survival online multiplayer mode
I would like to add 3 Characters
I would like to change 2 maps
I would like to add 3 weapons
Change Game name – Menu Choices
I will apply all required maps , assets , probs and characters
for android and IOS
my budget for this will not exceed 400$
** FPS GAME link will send you if we have agreement