Hi Rumpii,
1) Yes you can pull the float bowl off to check the main jets with out changing the gaskets, i've done it many a time and its normally a rubber O-ring. But if the gasket is damaged in any way when you take it off then you will need to replace it and make sure you done pinch it when you put it back together. Rule of thumb on how to up jet / down jet is that if the spark plugs are white then you are running too weak and risk over heating the pistons etc so will need to put bigger jets in, if the plugs are mega black and sooty then your too rich and risk detination and melting pistons so need to down jet.
2) there is no special paint for swing arms but hammerite is a cracking paint and normally comes with a 8 year garentee and is very hard stuff.
3) Tyre size should be on the tyres that you have on the bike (eg 190-90/19)
4) Try http://www.wemoto.com/bikes/yamaha/dt_125_re/05/