You can think of it in terms of moments about your aircrafts previous CG position:

You need to know the position of the old CG, the weight of both engines and the weight of the servos.

Increasing mass (engine) increases moment.
Increasing moment arm increses moment.

The nose down moment of the engine is it's mass (oz) x distance from CG (inches)
So if your old engine weighs 5oz and is 10 inches forward of CG it's moment is 50in/oz

So if the new engine weighs 8oz and is in the same position it's moment is 80in/oz.
(8oz x 10inches = 80in/oz)

You have increased the nose down moment by 30in/oz.

It follows then that you need to move the servos enough to create a tail down moment of 30 in/oz.

Find out the servos current contribution to the tail down moment: weigh them and measure their distance aft of the CG.

Let's say they weigh 2oz and are 8 inches back from the CG = 16in/oz

You need them to increase that by 30 to 46in/oz so to get 2oz to provide 46in/oz you need to move them to 23" aft of the CG.

46in/oz divided by 2oz = 23 inches.

Then by moving the servos you have increased the tail down moment enough to compensate for the increase in engine weight.


You can put this all into a formula

Servo moment arm required = ((e2W*eMA)-(e1W*eMA)+(sW*sMA))/sW

where
e1W = engine weight 1
e2W = engine weight 2
eMA = engine moment arm (distance from CG)
sW = servo weight
sMA = current servo moment arm (distance aft of CG)

Using the above as a worked example:
e1W = 5oz
e2W = 8oz
eMA = 10 "
sW = 2oz
sMA = 8"

Servo moment arm required = ((8*10)-(5*10)+(2*8))/2
= ((80-50)+16)/2
= (30+16)/2
= 46/2
Servo moment arm required = 23"
Which requires a movement of of 17"
(23-sMA)

Hopefully this should work for you!

Jon