1) you could keep the 7/900 series rear mounted dizzy but bear in mind that long term this is an expensive option. The rear mounted dizzy cap, rotor and leads will set up back over £100 a pop. Converting back to the side mounted dizzy will drop that cost down to below £50. To convert you will need tofit the intermediate cam from your B19 or from another 360 or 240. To blank the dizzy hole off in the back of the head you will need the blanking plate from any of the B2xx engines from either the 360 or 240 series.
2) If you are going to keep your carb then it will on bolt in and work. The carb realy will want a new needle and jet - use one for the B23 and make sure you fit a new diaphram at the same time.
If you are wanting toconvert to fuel injection then there is a fair bit involved, you will need all the sensors wiring loom and ECU's from the donor car.
You probably could blank the hole with a core plug but using the propper part from a later 360 or 240 would save hassle and you know it will work first time.
I take you have a LPG mixer on the carb not an LPG carb?
You can use the cam from any B19 B21 B23 or from a 240 or 360 B200 or B230 engines. It is only the engines fitted to the 7 and 9 series that did nothave the drive on teh cam.
You might need to change the drive wheel if your B230 is a round tooth geared rather than square tooth like the B19. It is all bolt in and no real messing about.