Gurus of all things Polaris,

I recently acquired a garage sale special 1993 Polaris 350 2 stroke that did not run. Previous owner said the HOT light came on and he had to drive it further but it died shortly thereafter. Pulled the cylinder and head off and found the piston had transferred some aluminum to the cylinder sleeve and was toasted. Honed the cylinder, put in a new piston (cylinder was at the limits for max size with .050 piston), rings, wristpin and bearings, Boysen reeds etc put it back together and started first pull but it's rather noisy. Sounds like a heavy rattle in the lower end which dissipates with RPM but otherwise runs great and has excellent compression. Drove around the block but the HOT light came on again and the radiator was cool (cylinder head was hot). Pushed it home and started it again later and noticed radiator had plenty of fluid but at idle no fluid was flowing through and this thing has no thermostat etc. Hot light switch checks good on cylinder and fan switch on radiator works as well.
Should fluid be visible moving under the radiator cap if the water pump is functioning (I did not find any blockages in the cylinder and block)?
Should this thing rattle like a metal milkcrate drug behind a truck (what does a loose rod sound like and/or piston slap vs normal 2 stroke sounds)?
How hard is it to pull the case and change the pump impeller etc?
Has anyone had this problem before?
Any idears before I sell this to some poor soul?

Thanks from a Polaris newbie.