Use a 3/16" drillbit and drill the head off the Seized bolt, the threaded stud should turn out either by hand or using Vise Grips.
Now to solve this problem,
I see this problem frequently here on the Forum and when around Bike Owners.
The problem is caused by Stainless Steel Bolts threaded into Aluminum Cases. The 2 dissimilar metals actually react against each other. The Aluminum becomes a "Sacrificial Anode" to the Stainless Bolt. Because of the properties of the 2 different metals. What is a bike Owner to do???
When you change the tranny oil, always, and I mean always use some kind of "Anti-Seize" compund. It is sold in most Hardware stores under that name. Others are Copper Coat etc. You can even use Plumbers Thread Compund, a sorta "stop Leak" for screwed together pipe fittings, or you can even use Silicon Gasket sealer. Simply wire brush the bolt threads to clean them of any corrosion, and then use some carb cleaner or Brake Kleen to dry any oil etc from them, then coat them "lightly" with the "Anti-Seize" material, and NEVER have this problem again. Most Hardware stores have a selection of Stainless Steel bolts, replace the Factory Torx head bolts with Allen head Socket Screws, they have a version called "Button Heads", they polish up real nice before installation.

 

Are you certain it isn't the zinc screws that galvanize themselves to the aluminum? Even if the heads are chromed. I have never had a problem with stainless steel. I agree the screws polish up real nice. I use a rouge wheel and silver polish in my grinder and the screws look better than chrome. And they don't pit like the chrome screws. I replace any screw I visit, except of course the ones that require hardened bolts.

 

The anti-seize compound is a good idea if installing stainless Allen bolts. Just takes a second. The pisser is Harley installs the Torx T27's so tight that the first person servicing the bike strips the heads if they don't use an impact tool. I suspect it's another way to get people into the dealerships for service dollars.