I have a 3800 solex from 1970 that i have been restoring the last months, but I forgot to check the fuel tank (plastic), today when I was ready to start the engine and filled it up with gazoline I saw that it leaked.

Is it possible to fix this or must I buy a new?

There is no hole in the tank, the leak seems to come from the seal from the two halves of the tank.

How can I reseal the tank? How do I separate the two halves and seal them again, what glue do i use?

It is possible to seal a leak but you have to be very carefull
he material is difficult to glue, you have to experiment
may be an infaltable boat repair glue can do the job.

A good way to repair: first detect the exact leaking area by submerging the tank in water and apply a little air pressure and check where the bubbles come from.
Then you have to be very carefull in degreasing the tank hot water and caustic soda ( washing machine pills ) should do the job, make sure that the crevice at the leak also is lubricant/oil/gasoline free.
Then use a smal electric soldering iron and gently melt the parts together. ( a little massage of the plastic to mix it may be necessary)

I had the same problem. leaks between the two halves. I tried but it failed to work. bought a new one at solexnoord.nl
imitation fueltanks are not that expensive.

Use coating in the tank, and he is dense.

Not for plastic, only for the steel ones.
You may end up with loose debri if you try to internally coat a plastic tank