hi montesa cota 200 175cc ran from 1980 to 84 mh been the later model as far as i am aware the cota 200 frame number started 29m or a cota172 19m (74 to 77 ), i too have looked for a drain and have only found one on right clutch case but the frame prevents it from been undone ,, it is best to seal tank inside with a ethanol proof sealant look on ebay, as for mods ,mine has a lengthened swing arm and am thinking of fitting pegs further back and slightly lower,as for fuel when you say gallon is that 4.5 litre or 5 litres ? or a full plastic type fuel can ? as theres a big difference ,you need to work on 5litres (5000ml) at 100ml oil = 50-1 ok for the mont if semi syn or fuuly syn the rev 3 could use slightly less oil ie 90ml to 66ml = 55 to 1 or 75 to 1 i use 90ml to 5litres in my rev3 and 100ml in the mont google montesa cota 200 pics and you should find plenty