Replacing the o-rings ,while a bit of work, will obviously be a good investment for the future. I suspect I will be doing the same for some of mine that haven't been run in a while. Good luck!
At least it is an inline 4. I perceive those to be easier to re-locate when completed than a V4. It will be all good. CMSNL or Web!ke for orings and things. There will be tariffs but you will get exactly what you want. You could also cross reference part numbers and see what is available in the US. Either way you got this!