If youre able to braze - ,a simple can will make a muffler -here are a couple of examples .The main thing on this design is to eliminate any baffle close to the end of the head pipe . These are simply expansion chambers.
Not knowing what the can is that you have - there is no " quick" guess as to what will work
as far as the Y from the engine - make these equal length into one pipe - and use a head pipes as large in diameter as largest dimension of exhaust port (close enough )- this will eliminate restrictions at port. Most headers are a bit larger in area than the exhaust port -which is also "close enough".
As previously noted --the tuned lengths are specific to rpm /volume and type of power band desired - The common error is to read some "theory of tuned pipes " and then try to apply that data for a aerobatic or sport use powerplant
a specific tuned resonance can wreck smooth throttle response and make for a very balky engine performance.