DT125 RE 2006 exhaust

[untitled_warrior]

recently i removed the standard exhaust from my dt to respray, after re-attaching it became evident that it hadnt sealed correctly to the exhaust port neither had the chrome tube attached to the top of the expansion chamber.

Now i didnt replace the gasket and am aware its more than likely a symptom of my problem

On acceleration the bike bogs down as the revs reach a certain point so can all this be solved by replacing the gasket?

my second query
on the exhaust there is a chrome tube coming from the top of the expansion chamber, what exactly is it for? (picture below) and why wont it wont seal / connect properly back onto the exhaust.

https://imagehost.vendio.com/a/25226091/aview/1253012018027_DT125_X_030.jpg

also what is the likely damage if any to the bike if rode under the above circumstances ?

any help much appreciated

thanks