I'm not sure what you are asking. Do you own a weird-running OS 25 that is 20 yrs old & your friend also has a just-broken-in one that he will sell you for $30? Or does your friend simply have a 20 yr old just-broken-in OS 25 that he will sell you for $30?

In either case, I think that $30 is too much for a 20 yr old OS 25, just-broken-in or not (unless it has collector value).

I don't think that you will have any particular problems in getting it to start & run, but it is probably a cross-flow design that won't be as powerfull as current engines.