I take it you are referring to the input shaft? If you are not running a clutch it is possible to cut off the smooth part of the input shaft but you should never cut the splines. What type of motor shaft adapter are you using?
My VW shaft adapter consists of a Ruland shaft coupler and the input to mainshaft coupler from a late model VW bus that was turned down to 1.125 inches in diameter and had a key slot cut in it. I cut off the pilot bearing section of my input shaft.
The Beetle doesn't require a rear motor mount. I believe the Bus and Type 3 do require a rear mount to be fabricated. The motor itself can usually be supported by just the shaft end, though I am not familiar with the Kostov motors. The Prestolite MTC motor in my VW based beach buggy is only supported by the transaxle.
My VW shaft adapter consists of a Ruland shaft coupler and the input to mainshaft coupler from a late model VW bus that was turned down to 1.125 inches in diameter and had a key slot cut in it. I cut off the pilot bearing section of my input shaft.
The Beetle doesn't require a rear motor mount. I believe the Bus and Type 3 do require a rear mount to be fabricated. The motor itself can usually be supported by just the shaft end, though I am not familiar with the Kostov motors. The Prestolite MTC motor in my VW based beach buggy is only supported by the transaxle.