90 hp 3 cyl merc question 2003

[falcon0310]

Guys can you help on few points
1 bought parts engine for block around 1992
Are they same apart from electrics and carbs ?
2, parts engine was stored for 8 years.
Checked compression all equal but only 90 psi but it's new tester could be out from machine mart ?
3 the 2003 engine has bigger studs that the other older engine for bolting on to mid section does that matter ?
Thanks Adrian

[Davie]

There are different blocks using different crankshaft and pistons. Get the two serial numbers and cross reference parts to check.