Changing to rigid fork on C-dale Killer V 900 w 1.5
Yes, you can use a 1 1/8" fork with a 1 1/2" frame. I did it on one of my bikes. You just have to get a special "reducer" headset. It doesn't look goofy at all - you can't even tell unless you look closely.

You'll have to get a threadless fork. Nobody makes reducer headsets for threaded forks.

Most new forks come with an un-cut steering tube - that means it's a few inches longer than most people need, and you have to cut it to the right length.

Make sure that the crown-to-axle distance is approximately the same as your old fork. If not, it'll change the bike's handling characteristics, maybe drastically.