Hi Babr, an interesting machine you have there. As for belt size, no they are not all the same. If I recall correctly there are three different belts used, the 98-2001 (X to K01) uses a different belt to the 2002 to 2006 (K02 to K06) and the K07 to at least L02 use yet another sized belt.
One of my bikes is a Y 400 and I have tried an assortment of belts from different suppliers. Currently I am using an OE Mitsoboshi belt (no its not spelt wrong before you ask and assume it to be Mitsubishi
I have also used cheap belts from the likes of Brooks Barn without
a problem, although for the price of £20 I have opted to change them each year but then 8,000 miles per year on a 'cheap' new belt when 99% two up is quite a tall order I suspect?
My worst belt was one supplied by a Chinese motor cycle parts supplier in the UK and I suspect the Belt was for perhaps the k2-k6 models as it was rather loose and flapped around. It did not break but it encouraged transmission snatch.... so that belt was discarded early on..
I have always fitted my own belts and for me, cheap belts with early changes 8,000 and not 14,000 miles has suited our riding style up here in Scotland. The last thing you want is a broken belt when out on a single track road, miles from anywhere when touring in the NW of Scotland..
I not cannot comment on clutch setups and personally would experiment with different weight rollers before moving to the clutch... unless the original clutch was worn out ? Of course you never mentioned the trikes weight compared to the X model scoot with two people in the bike? Just remember the X/Y model Burgman was marketed as a luxury commuter... as stated on the handlebar covers and its not a sports machine but having said that its no slouch
Oh, keep an eye on the engine oil; level.. it does vanish on some machines and has been the cause of quite a few wrecked engines. Not knowing how your engine will sit, you could find the dipstick giving a false reading