Vz800 crankshaft

Bit of a long post sorry in advance,i have a 97 vz800 marauder the crankshaft was wrecked so was told a vx800 would fit i bought one fitted it lined timing as per manual engine turns over by hand no problem fit spark plugs and connect starter direct to battery while on bench to test engine gives half a turn as if battery is flat ,I've fitted a new starter and tried with 2 different fully charged batteries it's as if the starter can't overcome the compression ,i did fit it back in the bike and with a lot of mucking around had it running on 3 different occasions the engine sound fine when running but turns over like a flat battery I'm pulling my hair out or whats left of it .