is there such as thing as a tapered-to-straight adapter for MTB headset?
Yes, many headsets made for it come with a crown race reducer. Cane Creek, FSA, Nukeproof and several other big names make them so that you can convert a tapered frame to accept a standard straight 1-1/8&quot; fork. Just be sure to take note of what type of bearing/headset your frame uses whether it be external, zero stack or integrated.
You can also likely buy the crown race reducer separately to be used with your choice of headset. A google search for &quot;reducer crown race&quot; will net you the results you are looking for.