cassius maximus

i have a tzr as a project bike and i want to make it really special.ive had one before,so i kinda know what im doing, but there'sa few urban legends i would like to staighten out.does the rear facing cylinder off a tzr250 vtwin,slot straight onto the 125 bottom end,without any mods?apparently it does, which would mean i could develop an under seat exhaust more easily.i also need to find a lighter flywheel/generator,and if anyone else can help with ideas(especially some formulas for exhaust design) any help would be much appreciated!!!thanx guys