Hi Sven, Tipsy Nippers and engines, eh?
Sorry if your engine problems have taken the fun away from your flying. I hope the following does not sound patronising - NOT my intension at all.
Your situation of having inherited an engine that has sat idle for many years is not desirable. Not even a motor that has been appropriately inhibited is immune to issues after a few hours of operation. Old fluids in an engine can cause, amongst other things, corrosion, blockages and gumming-up of springs and piston rings. Unless those issues are addressed, there will inevitably be issues down the line.
The importance of keeping a hot engine ‘pulling’ after aerobatics is well known and applies to glider towing aircraft too. If, after a period of running the motor at full power, it is then allowed to idle, then there is a genuine danger of severe damage to heads, cylinders, and other parts caused by rapid and uneven cooling. That is one of the reasons why so many like to install a CHT sensor onto # 4 cylinder of a VW; it helps to give a picture of how hot the warmest part of the engine is. But it is just a clue. Oil temperature, although helpful, will also vary and depend upon the oil type, oil cooler, OAT, how hard the engine has been driven, as well as the general condition of the engine.
More generally the problem of VW engines/Tipsy Nippers/aerobatics has been around for many, many decades. The ‘Master’ is Barry Smith. Sadly, Barry is long retired and no longer involved in flying, but he visited the issues and preached to the great unwashed as to affordable VW engines for decades. Trouble is, too few listened at the time. Anyone who owned or flew behind one of his 1.8 or 2.1 litre VW Acro engines will tell you how good they were - never an engine cough when the aircraft was rolled to the inverted… or stayed there! https://acro.co.uk/
Assuming you want to operate within the realms of the flight manual and stick with positive ‘g’ manoeuvres. You will need to install a VW that is not too heavy - that way won’t be coming up against the max aeros weight limit or getting your C of G outside the permitted range either.
To do so, you need to consider a light-weight ignition system, a suitable carburation and inlet setup, and oil cooling. You also really need to keep all the above under a streamlined cowling.
I would suggest that 1.8 l engine with aluminium heads will keep the weight down. LeBurg / Surefire ignition systems are the lightest you will find. And when it comes to carburation, the Zenith CD 150 installation with suitable jetting requires a simple induction system that then only requires a hot-air box to be added to it. Note though, the existing Ardem 1600 pipe ID is too narrow for the 1.8 l engine and may also require a different take-off attachment plate too.
To the above, add an oil cooler and oil filter, oil and CHT gauge system, a small modification to accommodate the carb under the cowling, and you are almost there. Finally, consider if the cylinder/engine baffling and sealing is in good condition. From memory, G-NIPR is one amongst many with the above installation; it works!
If you are thinking about an inverted oil fuel system… ask yourself if you want to waste years trying to perfect such a system or if you want to go aviating and just having fun in your Nipper!
Pictures below show an oil cooling and filtering installation.