我们还没有找到一个人替代他.we haven't found anyone who substitute for him

We can't find anyone to take his place by now.
take one's place
取代…人
你翻译的稍微有有问题,
we haven't found anyone who 【could】substitute for him
有一个could即解决了substitute时态问题,也代表一种语态,没有人能够代替他.
虚拟语态,表示与现在现实相反的意思.