Is Cessna 150 with incorrectly marked ASI airworthy?
Take a look at the equipment list of the Cessna 150.  There are part numbers for every piece of equipment installed.  If the part numbers do not match, then the airplane does not comply with its type certificate data sheet and is not airworthy.  Take a look in the maintenance manuals and find the part numbers.  If the rental FBO refuses to let you see those manuals find a different FBO to rent from.
Unfortunately, that is not the whole story.  There are also other ways for this to be an airworthy airplane:

PMA part.  If the new airspeed indicator has been approved as a direct replacement parts manufactured approved part, it can go in along with the equipment necessary to make it work.  I doubt this is the case as the part would have to be FAA approved for that airplane.  The most common application of a PMA approved part is the installation of LED bulbs for incandescent bulbs.

STC.  The airplane has a supplemental type certificate that modifies the airplane in such a way the new airspeed indicator markings are required for the STC.