massey 12 ?'s

hi have a few questions i own a maf 12 garden tractor its a hydro transmition and i wanting to change the hydro fluid filter but i cant find a filter for it the one that mf made for it is no longer available does any one no where i can get one and another ? when i got the 12 it was smoking alot and i found out the oil seal on the pto side of the motor was leaking i put a new one in it dosent smokke as bad but it still uses oil real bad and my dad sugessted i get a briggsn stratton vtwin 16 horse motor in it i heard techumseh is in a little problem with keeping the company going but the thing is i dont know how tought the tranny is does any one know if its ok to put like a 16 horse engine on it or not and if i do how long would it last from going from 12 to 16 id like to keep the hh120 on it but i just never have time to over haul the thing just wondering if any body knows