I`m still confussed! It was perfectly normal for me just to measure the stem diameter and try and get another one in better condition. The forks have an 11/4" threaded steerer . :?
have a look at the old marin catalogues. marin indian fire trail frames always had a 1 1/8" headtube.
if you want an original marin lite stem have a look at ebay. they show up there quite often. a tioga t-bone or ritchey stem would be nice too.