Well, the engine needed to run a chainsaw is pretty small. Taking into account millenia of technological advancements, it seems a likely scenario that if everything were to remain based around combustion type engines that they've just been made smaller and easier to start. Really, the power of modern day chainsaws is enough to hack through flesh just as easily as they show in the trailers of DOWII. Even in books they describe the SM having to give it a good yank to untug it from an enemy. I doubt fuels have changed all that much because with hundreds of millions of worlds worth of resources to exploit, gas would be probably around a penny a gallon or so. This especially seems the case as even the land raiders, rhinos and other vehicles all have the sound of typical combustion engines not the humm of electric or whatever sound nuclear reactors make. Though, they have learned to harness plasma as a power source.
"There is a cancer eating at the Imperium. With each decade it advances deeper, leaving drained, dead worlds in its wake. This horror, this abomination, has though and purpose which functions on an unimaginable, galactic scale and all we can do is try to stop the swarms of bio-engineered monsters it unleashes upon us by instinct. We have given the horror a name to salve our fears; we call it the Tyranid race, but if it is aware of us at all, it must know us only as Prey."