fitting Clarke 3.2 tank to a '03 ec250

[flybars]

I've searched and searched, but couldn't come up with answer's to my questions. The tank bolts up fine. With the tank off, it exposed a square cutout in the top of frame, with a m6 threaded hole below that, tapped into the frame down tube. Should I go ahead and plug that little hole? Can dust and water enter and do damage?
Also, I hope I can use the seat/tank post from the stock tank on the Clarke. I'm concerned that my seat is way too long to fit the bike now too. I'll find out later when I get everything bolted up.
Can someone who's gone thru this give some advice. Thanks in advance. Headed for the hardware store to find all the fasterer's I'll need. Half the faster's on this '03 have mismatched bolts, washers and screws. But I love the bike!

 

[Cruiser]

That hole is where the voltage regulator went on some of the bikes..

 

[flybars]

Thanks Cruiser. I was able to find all the fastener's I needed at a local True Value hardware store. Everything bolted up well with two discrepencies. The first is when I tried to fit the stock '03 seat, it will not fit 'cause it has too long of a nose at front. But the second seat I got with the bike looks like the 'o5 seat I had on my FSE450, and fits well. I need to order some new foam and a seat cover for it. And the second thing is that all my pan head screw's are completely exposed with the seat sitting higher above the rad shrouds. I can live with that tho. I'm very pleased with the extra mileage I'll be getting.