Elec start problems

Seeing as it didn't do anything with the old battery, then turned over but "jammed" with a brand new battery then I'm guessing here but it sounds to me that the new battery is also not fully charged (just cause it's new doesn't mean it's fully charged, may have been sitting on the shop shelf for god knows how long).
I'd try jump starting it off another battery (12v I assume) &/or fully charging your new battery.
The high compression of the new motor is just too much for a poor or slightly discharged battery.

If kicking it over is too difficult then try kicking it over and hitting the elec start at the same time (with a full charged battery). It is way easier on both you & the elec start.

Just my 2cents & I'm no expert. Someone else may have a better idea of what's going on.

robatik,
are the decompressors (valve lifters) on your 'new' engine fitted, and adjusted to work correctly? The most important for e-starting is the one mounted in the camshaft. This is probably a key point, is not wonderfully designed and does go wrong from time to time. A fully charged battery and correctly adjusted valve clearances are important too, and it is also possible you will need to look at improving your starter motor if this engine is super-healthy. There's a lot of excellent detail in the 'doc' of how to fix your decompressor and up-rate your starter. It could do with a good bit of editing, but everything you need is there and lots of additional related stuff on the forum.

Cheers... Paul