Hello,
I am not as experienced as everyone else on here but I can tell you that I was having issues with my RC Valve motor and I think, besides looking for a used one on Ebay or even Jauce the Japanese version https://www.jauce.com/search/Honda+NSR250R+MC18+RC+servo the best option to get a working product is to send your servo to T2 Racing who will exchange it for a good one within two weeks of receiving yours.
I am in the middle of that process. I sent mine to Japan a week ago.
Contact Yukako Hanakawa via e-mail: shop.t2r@gmail.com
Also Ebay has some but of course we don't know if they are any better than yours! https://www.ebay.com/sch/i.html?_nkw=1988+honda+nsr+250R+RC+Valve+servo&_sacat=0&_from=R40&_trksid=m570.l1313&_odkw=1988+honda+nsr+250R+RC+Valve+servo&_osacat=0