The Velleman amp is rated at 70W into 8 ohms or 100W into 4 ohms. Add a 2nd set of output transistors then it might provide 120W into 4 ohms. You won't notice the small increase in power.
Since it is already rated for a 4 ohm load, then using more powerful transistors and heatsinks or doubling them, using a more powerful transformer and increasing the trip point of the current limiters, then it will double its output power to 200W into 2 ohms. You probably cannot buy 2 ohm speakers so use two pairs of 4 ohm speakers in parallel.