A waist coat is easy to make from scratch. Joann's has patterns on sale all the time. Make a mock up of it out of muslin ($1-2.00 a yard cheap fabric) before you cut anything out of good fabric. This way you can find out of you need make any special adjustments for your kilt, or to fit your belly. (When i make my husbands I buy a pattern to fit his chest, then do a special alteration to accomedate his 'bay window'. He looks much sharper that way.

As for trying to retailor a coat when it is big enough in the sleeves but too big in the front, that is a major reconstruction. I'm not saying it isn't doable but it is a lot of work.
Check out a tailoring book from the library for reference if you attempt this. Another help is "Reader Digest Guide to sewing" look for the 1980's edition. All the later one's they took out the tailoring stuff.

let me know if you need help. I'm not great but I might be able to explain some stuff to you.