Can you rip the full sized architrave lengthwise until it does fit? Then if your rip cut isn't perfectly flat, a plane will help take out the high spots then just caulk the join to the adjacent wall.
Not impossible to rip the architrave with a hand saw. Take your time and get as close as you can, then as stevo said, caulk or fill the gap. Have you done any sawing before?
Can you rip the full sized architrave lengthwise until it does fit? Then if your rip cut isn't perfectly flat, a plane will help take out the high spots then just caulk the join to the adjacent wall.
Filler and paint makes me the joiner I ain't.