You can use socat.

Nginx config:

location / {
    proxy_pass http://127.0.0.1:81/;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $remote_addr;
}

Socat command:

socat tcp4-LISTEN:81,reuseaddr,fork,keepalive,bind=127.0.0.1 SOCKS4A:127.0.0.1:yourtordomain.onion:80,socksport=9050

You can use socat.

Nginx config:

location / {
    proxy_pass http://127.0.0.1:81/;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $remote_addr;
}

Socat command:

socat tcp4-LISTEN:81,reuseaddr,fork,keepalive,bind=127.0.0.1 SOCKS4A:127.0.0.1:yourtordomain.onion:80,socksport=9050

Update:

If you want run socat as daemon. Create file socat.sh and set permission to 744:

#!/bin/bash
socat tcp4-LISTEN:81,reuseaddr,fork,keepalive,bind=127.0.0.1 SOCKS4A:127.0.0.1:yourtordomain.onion:80,socksport=9050

And run it over screen:

screen -A -m -d -S SOCAT_DAEMON ./socat.sh