Hi Mole,
Fitting a new pullstart is quite an easy task as there are only 4 screws that hold it onto the back of the engine, undo those and the whole thing will lift away from the engine. There is a Hex shaft inside there as well which you need to keep as it is that part that engages the pullstart to the crankshaft in the engine.
There is a separate backplate on this engine, the one you will see when you undo the pullstart. Whilst you have the pullstart off, have a look at that backplate and there will be another 4 small screws there that secure it to the crankcase, make sure these are tightened up fully, use some thread lock if you need to, as these do undo from time to time through normal vibration when running. If they come undone you will lose pressure in the engine and have leakage which will cause erratic running. (I have recently tightened mine up because of the same problem)
I don't think you can do a roto-start attachement on these and even if you could, you have the problem of accessing it once the engine is mounted in the car. There are other parts of the chassis that will be in the way.
As for the slippage on your current pullstart, it sounds like the spring has gone inside where it secures into the plastic housing inside the pullstart, you can buy separate springs but they can be fiddly to replace, a whole new pullstart is not much more in price and a lot easier to fit.
If you do buy one, follow the steps above and simply slide the hex head of the rear shaft into the pullstart and then fit the shaft in the hole in the backplate. Once on simply replace your 4 small screws and screw the plastic housing onto the back plate, (I always use thread lock on mine now) and that is it....Job Done
The whole job can be done within 10 mins and that includes taking the engine out of the car to do it !! It is really easy.
Regards........Mark.
maybe i should....