Why .040" over? Heave you measured taper? Go least oversize required to make it straight and remove any damage. .030" gives you the most options for pistons, sometimes cheaper than other sizes.

I see the $450 ebay kit... I wouldn't touch it without having a list of brand and manufacturers PN of each component. Probably fine for slapping together a stockish low output engine but I wouldn't use it. You're paying for machine work, don't waste that money by saving a few hundred on decent pistons and other parts. To cheap out like that I'd sooner pull it apart and as long as taper wasn't ridiculous just hone it as even as I could and slap new rings and bearings in it.

I'd be tempted to run TFS 170 heads, good flow and reputation from what I read.
tfs-51410002-m61 - $1125

These head gaskets are smallest compressed thickness (.040") I saw at a glance suitable for marine use.
fms-m-6051-cp331 - $77

These pistons would be my choice at a glance. Forged, suitable 14cc dish for those 61cc TFS chambers to get you .013" in the hole at factory deck height for 9.47:1 compression at .053" quench with those head gaskets. Not excessive, but would be better zero decked for 9.75:1 though that may be pushing compression a bit. I'll have to run dynamic compression with some cams and see where it falls.
srp-149606-8 - $620

if you want cheaper pistons you could run hyper KB-303. This puts is .023" in the hole for 8.79:1 at .063" quench. not much squeeze and getting close to detonation prone. Zero deck it and you're at 9.22:1, a good ballpark for a marine engine that will be running a mild cam and shouldn't be detonation prone.
kb303-030

I'll have to run some dynamic compression numbers and plug it in DD, but your goals are reasonable.