I dunno…if you are looking for putting different plates on a harp www.turboharp.com’s harps have lots of different covers you can get.
Other than that…most harmonicas are relatively uncustomizable. Other instruments you can get different pieces and parts for, but if you buy a special 20…the only plates you can put on it are sp20 plates…the only reeds are sp20 reeds…etc. I have never heard of anybody buying a ‘custom’ harp…unless they literally build all the pieces themselves.
But I don’t really know what you mean.
Maybe what you want a harp that you can emboss reeds, tune different, etc. If you’re going for that…really any harp will work. There are a few things that you can do to cover plates, depending on what harmonica you like. You can open the back up…bend it to fit your lips better, but again, I don’t know what you mean by ‘customizing’.
As far as getting ‘real’…I think you will find that a nice harp plays REALLY nice. My favorite is a Marine Band, and I play it all the time…but I can’t think of one thing I would change. Only thing might be changing the wood comb for a plastic one, but they don’t make plastic combs for that. If I want a Marine Band with a plastic comb…I pretty much just have to buy one.
Hope this helps!