Oh how well you know me Ajays .............
Smithy - first you need to decide WHAT you want to build - Cafe Racer, Streetfighter, Race Replica, Lowrider [ Chopper / Bobber ]..................
Each one has it's own demands on the basic Kay shape / make up.
F'rinstance - Streetfighter is the simplest and still allows a degree of individuality - ie noranothafugginbandit.
You can go with all sorts of tail pieces [ I used R1], headlamp fairing, upswept exhaust, Renthals, etc.,etc.,etc. Larger wheels ? No probs - K1100 3 spokes will fit K100's [ need k1100 forks ] and K1200 / R1100S will fit K1100. Rears a straight swap in, fronts require a modified spindle - I'm still working om that. That gives you a 5" / 5.5" rear with a 180 tyre - a bit more like it.
Lowrider / Bobber...... again no real problem - lower the whole plot, lower seat height, Wire or Cast wheels - The tank is the only problem and we're getting round that on my mate's version by making a false grp H-D style tank to cover the stock tank.
Race Rep - no probs - wheels / brakes as above. I am using an R6 [ '09] fairing - just as BMW did when they were developing the S1000RR. Did use a 916 fairing, but it looked "old hat".
Cafe Racer - several considerations, IF it's a 60's Brit style iconic look then you need Wire spoked alloy wheels [ IMO ], AJS 7R or similar style tank, ditto race seat. Clip-Ons chrome headlamp, and a suitable period silencer [ Gold Star?]. I've gathered all the parts for mine, just need to bite the bullet for the £300+ I need to have a period CR style tank made. Radiators are a bit naff [ although WERE around in the 60's] - so I've "hidden" mine behind the engine where the battery used to live 'ish.
A Japanese era Cafe Racer would be simpler as cast wheels were in by then.
Both pix are "work in progress". The Fighter is a little closer than this pic. 5" 5 spoke k1200 rear wheel in, just awaiting aforementioned spindle mods for the front - then it's a strip down, detail and paint.
CR is the collection of bits - just awaiting a tank resolution - I might just polish it for the time being [ alloy ] but that would be a bit of a cop out for me.