For reducing compression, I'd suggest having your head or piston milled instead of using a spacer. A spacer will mess up your squish and make your engine run less efficiently. Also, if you're still using the 125 head, switch to the 150 head, it's a bit of cash, but it will reduce compression, increase combustion chamber volume, I believe it has larger valves, and will keep your squish nice and tight for optimum mix turbulence and combustion. Port your head, too, the stock intake port ID is several mm smaller than the ID of the Polini intake manifold. That should increase both low and top end by decreasing intake resistance and creating a "funnel" to increase intake velocity past the valve.
Here are the measurements for the intake and port:
Polini intake manifold ID 25mm, cross sectional area 1963.5mm2
Head intake port ID 21mm, cross sectional area 1385.5mm2
That's quite a difference and if you have just a step down in diameter where the manifold meets the port, that's going to cause a lot of turbulence in the intake airflow.
You'll also need a new intake spacer/insulator, one off a GY6 with a ID of 23mm and bolt spacing of 45mm should work if you open it up to 25mm. There's ones on eBay for not too much.
http://www.ebay.com/itm/121487279162?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT