Certainly grammatically, and partially correct semantically. Enthusiastic and enthusiast both come from the Greek ενθουσιαζω, to be inspired by a god, and an enthusiast was once a term applied to what we'd now call a charismatic religious sect. Enthusiastic, i.e., full of enthusiasm, has lost its religious connotation and now means per the OED, full of "rapturous intensity of feeling".
An enthusiast can refer, as you surmise, to someone who is enthusiastic for, in, or of something in particular or in general, and the OED gives the following example from 1856:
The energy and sincerity of enthusiasts is powerful in all ages.
But the more modern usage of enthusiast is that of devotee of something, implying a keen interest but without the divinely-inspired fervor. Here's an amusing example from a 1996 issue of GASBAG (i.e., Gilbert and Sullivan Boys and Girls, a journal of the University of Michigan Friends of Gilbert and Sullivan Society)
I'm addicted to Lord Peter [Wimsey]
with his Birtish airs and monocle,
And Nero Wolfe, and Father Brown
(with morals quite canonical).
By far there's good old Ellery [Queen],
whose puzzles are confusiest --
In fact, I am the model of a mystery enthusiast!*
Here the singer declares his interest in mystery fiction.
*This is a parody of the patter song "I Am the Very Model of a Modern Major-General" from Gilbert and Sullivan's Pirates of Penazance. Peter Wimsey, Nero Wolfe, Father Brown, and Ellery Queen are fictional detectives.