I'm building up a 9 speed frankenbike for the winter. Using a existing crank with the small ring removed the chain alignment to the rear is too tight and the chain drops off the front to the inside. Is a single ring crank required (do these run the ring closer to the frame?) or can the ring be 'spaced' inbound?
9 speed - chain alignment
a front derailler is your friend.
get a shitty old one and using the stops (or a block of wood) set it up so that its tight on the inside. unless your running horizontal dropouts on a single speed you need something upfront to retain the chain.
You can also buy chainring spacers, or if your running an old-style b.b AKA not x drive you can get a b.b that will receed into the frame on the driveside (old racefaces for example off the top of my head)
yes, a dedicated single speed crankset will run a tighter chain line, but you could just use what you have.
blackspire makes a 28T chainring that fits in the granny position on common cranksets, available for common 4-arm (104/64) and old-skool 5-arm (110/74) cranksets.
if that's too low, you could try anything mentioned above, but spacing the middle ring inboard makes the most sense to avoid a weird chain line.
if its an older crankset with the spindle in the bb (ie. non external bb) then running the bb backwards or moving spacers to tighten up the chainline might work great, as mentioned. you may have to shave off the granny tabs to clear the frame.