I have been able to source Ohlins parts through Steve Beane at After Hours Cycles,
http://www.afterhourscycle.com/.
I have had him send me valving pistons, seals, seal heads, reservoir pistons, reservoir cap, etc., for the Ohlins which came on my 2002 EC 300. My guess is that he can find you a new one.
Not sure of the AL alloy used on that piece - hard to say if it can be welded.
Someone with a mill and a lathe could make one, but it might be cheaper / quicker with Steve Beane.
Steve is a member here, and he formerly sold GasGas before the "drama." His handle here is "twowheels" and he is a great guy. I have him chasing some Ohlins TTX parts for me as I type.
He's not been on this site since 2020, so it's probably best to contact him through the website listed above.