ercoupe performance

I've flown mine at Port Clinton, fully laden to 1400 lbs (me, camping gear, full fuel) and I still saw 500fpm easily. Mine does have a climb prop and 100hp, but yours should still manage more than what it does, so I'd have your mechanic give the engine a going over to see what's up. I'll see 1000fpm with just me and fuel.

Just recently purchased my ercoupe a 1946 415-c
it has been many years since I've owned an airplane so I've signed up for I suppose it would be called an extended by-annual review.
The engine in my plane is supposed to have been upgraded to 85 hp but the weight and balance is still 1260 lbs. there was supposed to be paper work with the plane
for updating to 1320lbs but I haven't found it. My physical has expired so I will be flying in the Light Sport category. Don't want to get involved in the burocracy.
My problem is with full fuel, myself 185lbs, and a skinny, can't be more than 120lbs instructor, my plane just will not climb. When practicing touch n goes
we can't even reach normal pattern altitude in the circuit. My intentions were to take as passengers several grandsons weighing more than me for sure
but don't know exact weight yet.
The airport where I'm flying in Port Clinton Ohio is at 600plus ft. and and pattern is 1600ft I'm lucky to reach 1300 or 1400 by the end of the downwind when
staying in the pattern.
What can I do, what can I check, to improve the performance of my plane?