David,

Forgot to mention, you can make clean straight cuts on a scroll saw that does not have guides by simply making your own guide.
I take a piece of hard block or a metal yard stick and clamp it to the scroll saw. You'd have to make a device or jig to make sure the front and back are even to your guide and the blade.
Then make a couple of practice passes and adjust as required. I've done it before and wont take long to figure it out.

Hope this helps,