In case your ~/.subversion/config file does not have ssh tunnel defined, you can override ssh command on per-call basis using SVN_SSH environment variable
SVN_SSH='ssh -p 20000' svn co svn+ssh://server.com/home/svn/proj1 proj1 --username jm
deo@fox:~$ SVN_SSH='ssh -v -p 20000' svn ls svn+ssh://svn/
OpenSSH_7.9p1 Debian-10, OpenSSL 1.1.1c 28 May 2019
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: Connecting to svn [192.168.177.2] port 20000.
debug1: connect to address 192.168.177.2 port 20000: Connection refused
ssh: connect to host svn port 20000: Connection refused
svn: E170013: Unable to connect to a repository at URL 'svn+ssh://svn'
svn: E210002: To better debug SSH connection problems, remove the -q option from 'ssh' in the [tunnels] section of your Subversion configuration file.
svn: E210002: Network connection closed unexpectedly
This won't work if your ~/.subversion/config file does define ssh tunnel, in that case config file takes precedence. (Which is rather unusual. Normally it's other way around, environment variables usually take precedence over config files)