The honest opinion of a Magnum flier...?
Why bother with anything less than the WEST engine?
Get that engine and bolt on a 9x6 for the first flights if you are insecure about your flying skills, and get someone to launch it for you. It will go straight out of sight on a 9x6... so don't worry about the launch...
The Magnum really flies very good, as long as the engine runs
, so don't worry too much about your skills.
After a couple of flights when you start to get used to it and have it trimmed properly... bolt on an 8x8, shorten the pipe a little (see manual)... and hold on...
But don't forget, if things get out of hand your first reaction should to throttle back... that goes for any plane...
If you really don't like going very fast for the first few flights, go with a Webra 50, since it is a drop in replacement... that means less worries later on...
When you use Parker screws to hold down the engine, a swap between a Webra 50 and the WEST shouldn't take more than 10 minutes. You can do that on the field without problems.
PS. Set the CG at 135mm and slightly reduce both aileron and elevator throw.