A new 60cc cylinder from Spookytooth Cycles fits right onto the cases of a 48cc Grubee engine and has the same bolt spacing for the intake and exhaust that the 48cc has. Here's its specs: .7mm deck, 135.7° exhaust duration, 100.6° transfer duration, 17.6° blowdown, 116.8° intake duration.
It is a bolt on modification but I recommend you at least do these 3 thing to it: 1) use an exacto knife to trim the included base gasket to perfectly match the base of the transfer ports since it sticks into their flow area. 2) smooth the transfer port roofs which have a nasty lip which would disrupt flow at peak rpm. Just use a rotary tool with a grinding disc. 3) Use a rotary tool to grind off the crankcase interior at the transfers so that it better matches the cylinders bases of the transfers.
Here's the specs on my test engine that I upgraded to 60cc: 14mm Dellorto carb with .6mm main jet, standard exhaust pipe with two small holes drilled into the end for less flow restriction, no .7mm head gasket, Jaguar CDI. The cranking pressure was 100psi and the top speed was 28.8mph (46.3km/hr) at 6530rpm.
I choose to modify the engine in stages to see what it would take to get significant power increases. What I found out was that all you need to do for a power increase was to increase the cranking pressure. If you want speed increases then changing the porting is necessary. Here's the results of my testing:
Stage 2 of the upgrade: I used the rotary tool to increase the port durations to 152° exhaust and 112° transfers and 120° intake, enlarged the exhaust manifold (see graphic), widened the intake and exhaust ports. With this there was a 3% speed gain to 29.6mph 6710rpm with the same broad range power.
Stage 3 of the upgrade: I lathed 3mm off the head mating surface and installed the head gasket. This resulted in 120psi cranking pressure and 32.4mph (7340rpm) which was a 9.4% gain with better power throughout the rpm range. It was lathed till there was only .5mm "rise" in the mating surface. The unlathed head is on the left.
Stage 4 of the upgrade: increase the cylinder port timing to 161° exhaust and 121° transfers with 1.2mm transfer ramps dremeled into the piston at a 60 degree angle from horizontal, increased main jet to .69mm. result: 33.2mph with good broad power
Stage 5 of the upgrade: added 3 inches to intake tract using JNMotors offset intake and 2.5" hose for a total tract length of 7.5": 35.3mph (+6.3%)
Stage 5 of the upgrade: install a lighter weight piston wrist pin with a 7.5mm inner diameter for better engine balance and less vibration. result: +2mph
Stage 6 of the upgrade: install Rock Solid reed valve, hole the intake skirt of the piston, dremel boost port in cylinder.
Stage 7 of the upgrade: replace standard exhaust with Jaguar Torque Pipe.