bleach1

I have an Os 12tr that has run around 1.5 gallons of fuel through it now in the last 1.5 years. Now the engine still seems to have enough power in it and runs quite nicely but i just wanted to know what sort of maintenance to do on it and when to do it. The only problem i have had is it seems to be leaking between the carb and the block because its always very dirty there are a run. I removed the carby but accidently knicked the metal on the carb. (i used some auto sealant to fix it i hope). Is this a common problem with this engine and what should i look at doing with an engine around this age?